I'm guessing your son will do very well right off the bat. I'll start this by saying a lot of times - especially with the outgoing personalities, the parents are way more freaked about changes like this than the kids are. If you don't treat like a big deal (but also don't surprise him with it - go ahead and talk to him about it) he probably won't think it is that big a deal either. It is a good idea to start addressing the change now and just continue to bring it up somewhat casually and with a bit of an "oh this is going to be so fun and exciting" kind of tone occasionally throughout the next (stretch of time) so it doesn't come as a shock, but also don't expect him to be upset by the change either (sometimes we create our own problems with our own negative expectations and outlooks - they are far better at reading our tone and knowing when we are stressing about something than we like to think).
If he is joining the school at a time when many of the kids are new to the school I wouldn't worry about any of these first two categories (except the bathroom skills), but if he joins when most of the kids in his class have already been at the school for awhile you may find these first two categories helpful.
Essential Skills to Have in Order to Attend Confidently (At most schools)
A Caveat - no preschool child is perfect at any of these yet, and teachers expect to continue working on these. However, children should generally have some of these skills and exhibit them more often than not in order to not be "in trouble" all the time. These are the kinds of skills that if problematic will be brought to your attention to deal with right away.
Going Potty and Bathroom Skills: This varies slightly from school to school (even classroom to classroom depending on the age of the kids within it) but at many more formally organized preschools, children are expected to handle their bathroom skills supervised but without a lot of assistance. Things such as wiping, washing hands, pulling pants back up. . . are all things the teachers will help with when a child is having trouble, but they expect not to have to help with these things regularly. When I was in a three's classroom (where they were expected to already be potty trained) I had a student that had never even tried to wipe for himself upon arrival to the school - it wasn't a big deal, but the first thing we did, was talk to the mom about how to help him learn to do it for himself (that same day) and we started him on a plan to achieve self-wiping within two weeks of arriving at school (who-ever does the laundry at your house, might want to prepare for seeing some skid marks from time to time). If the school does potty training, they are less likely to be as strict about this in classrooms where there are children who are not yet potty trained, but independence in this area is just a good thing as soon as possible anyway.
Social Skills: Things like sharing, taking turns, waiting for the teacher and listening will be practiced and there will be goals toward improvement in areas where a child is weak, but while teachers will expect to need to prompt kids, help with conflict resolution and the like, they will expect any child to have had Some experience in these areas before attending. As your child has already attended a day care, (for parents of children who have not attended daycare, don't worry, play-groups, siblings, time with extended family. . . all count!) you probably don't need to worry about any of this.
Circle Time: We had a loose expectation that at two, a child should be able to participate in circle time (sitting quietly ready to listen, raise hands etc.) for about five minutes, by four the expectations was closer to fifteen (at the beginning of the school year, closer to ten and twenty respectively in spring). Again, this will vary from school to school and no sane teacher will expect perfection (especially at first) but some sense of being able to sit and listen to a short story/ participate in a calendar talk and other such activities will be something he will do on a daily basis at least once per day right away.
Skills that Are Helpful to Have (but most are not necessary)
Dressing Skills: In the US, I believe most states regulate there can never be more than 10 kids/carer present in the classroom until children are at least five. Your state may be different, but I believe that is still generally the standard. Because of this, preschool teachers can help with getting shoes emptied of sand after outdoor play, buttoning coats and other general dressing difficulties for kids this age, but if your son can confidently handle whatever clothes he is wearing, he won't have to wait his turn for help as much - a big plus when you are three/four.
Name recognition: Since everything from his cubby to his artwork will be labled with his name, it is great if your son can pick his name out of the crowd (first name and last initial) and even better if he can write it too. Again, this is not necessary - the teachers will work with him on both skills if he doesn't have them already, but they will be some of the first to be worked on.
Recognition of Certain Symbols: many classrooms do weather, how am I today and calendar talks daily. In these circumstances, the teacher will introduce the symbols and again, it isn't a big deal if he doesn't already know these things, but it can be helpful to your son if he has at least seen some of these things before. Things like what the days of the week are, what a calendar looks like, how a sunny day vs a rainy day might be depicted in a picture and finally (but less commonly) how emotions are depicted on "faces" made with a circle and a few simple lines (happy would be a smiley face, angry might have eyebrows pointing in) can help with the "how am I feeling today" can come up on a daily basis and feeling confident with at least some of this material will help him feel more a part of the group right off the bat.
For your son to be most comfortable and ready for the change, it is good for him to feel in control of the situation and not be surprised by it. Most kids that are outgoing don't need a lot of preparation for starting preschool other than just helping with an understanding of what to expect - and there are a lot of resources to help with that.
The first resource is the school itself:
Schools often have a system for inducting their new students into the school culture and expectations. As a spot opens up, you are likely to get a memo addressing this very question from your school with specifics your school has found helpful for its particular community. In any case, take their suggestions to heart - they come from experience.
Once you know a spot is opening up and have a time-frame with which
to work, see if you can have your son visit the school (if you
haven't already). Let him "tour" the school with you, ask questions
of the director or teacher that heads the tour, meet the teacher in
whose class he will be in and maybe even spend a half hour or so in
class with the other kids (if possible).
Get to know the daily schedule. Let him know what will happen when
he arrives in the morning, approximately when snack will be (relative
to other things in the schedule like outside play, circle time, drop
off. . . ) He doesn't need to know every detail, but if he has some
sense of the types of activities he will do, when snack, lunch, nap
and pick up are he is likely to feel more comfortable.
Books and Videos To Introduce Formal Classroom "Basics"
- While your son isn't starting elementary, there are a lot of things a formal preschool program (especially one that also has a K level classroom) shares with an elementary school. So, using books and videos intended for teaching about or addressing this milestone can be tremendously helpful in building confidence. LeapFrog's Let's go to School is an example you could check out if you'd like. I know it is available on Netflix Streaming. Little Critter's First Day of School is another good example from books. These are just two quick references with which I am familiar - there are a plethora of these, so please don't think these are the only, or even the best options out there. Your local children's librarian will be a great resource on this one. One of my favorites for "first timers" to being away from Mom and Dad is "The Kissing Hand." though it is a little long for a three year old - you may want to pre-read and "cut" if he has a short attention span.
Just introducing a few books and or videos like this into the mix over the next few months will allow your son to work through any general questions or concerns he might have in this "fictional" kind of way. It can help to open up conversation between you if he is worried at all too.
How you are a Resource
You probably already have a "drop off" routine as well as a "pick-up" routine. As much as possible, don't change these. If you've always given him a "monster kiss" and a hug before you left him at drop off, do the same thing at the new school and reassure him ahead of time that you will continue to do the same thing at the new school (unless he asks for a change - then of course, if its reasonable, go with it).
He may be concerned about missing old friends - where possible, set up some play dates and let your son see you exchanging phone numbers and email addresses with the parents of his friends.
Trust the Teachers
With small teacher/child ratios, preschool teachers are generally pretty able to get to know their kids pretty well. Build a rapport with your child's teacher as soon as you know who that will be, ask questions and be interested, but also know they are there to help your child and will be aware that he is new to the community. They'll help guide him through challenges in those first days and alert you to any problems if they crop up. You've chosen this school for good reasons and I'm sure you've done due diligence to be sure it will be a warm, welcoming and appropriate environment for your child. Relax in knowing that even if he has none of the skills I listed in the first two sections - he'll get there and not only will he have your help in doing so, but also a host of educators from the school as well.