3.5 is on the young end to learn to ride a bike, but not impossible certainly. The challenge a 3.5 year old will have is pedaling strength; you can help a lot here by having a lighter bike (most of the "Walmart specials" that 3.5 year olds tend to get are made from very heavy materials, not many people want to spend $200+ on a bike for a kid to ride for six months).
The approach that I recommend for kids this age who did ride a balance bike is fairly common, I think. Starting out, here are some preparation points:
- Don't use training wheels at any point. Obviously too late for that, but perhaps for someone else... my kids went straight from balance bikes to regular bikes, and it worked very well.
- Padding! Not just a helmet. Knee pads and elbow pads and padded fingerless gloves. This does two things: it reduces the chance of them being hurt, and it gives them psychological help as well. We're not talking serious injuries here of course - but little "ouchies" can slow things down a lot. Having padding, and telling them that the padding will make things not hurt, and showing them, make a world of difference. I actually had my kids fall on purpose before we started, so they could know that it indeed wasn't an issue!
Then when you're ready: start with the pedals off. Guess what that means the bike is? A balance bike! Let them get used to balancing on it. This can take a few weeks. The bike isn't the same as their balance bike - it's probably bigger and heavier, and it's just different.
Once they are comfortable with that, then add the pedals. No training wheels - even if you used them before, don't add them back. We had a friend who did this (had training wheels, removed them AND the pedals, then added back pedals) and it worked fine - just make a big deal of "You know, I think you're ready for pedals now - you're not falling off the bike any more. What do you think?" Let them "balance bike" a bit even after the pedals are on (feet on the floor still), just so they can feel that it's not really any different - a bit heavier, but otherwise the same. Then, don't forget to raise the seat a little - not a lot, but balance bike relies on foot-flat-on-floor while pedal bike foot, even for a new kid, shouldn't quite be flat; more height means more leverage!
When you add pedals, remember that the thing they're missing that helps keeps a bike upright is forward momentum. Particularly at 3, they're probably not strong enough to start pedalling. So - do that for them. Hold the bike from the rear - the seat - and sort of run up a bit with them, get them going while they pedal and you're pushing. You don't provide too much stability here - just a bit, but as soon as there is some speed, let off on that part.
Then, from here on out, what we did was measure distance. For my two, we went to a sidewalk, and I'd let them go and then count how many sidewalk squares they could go before stopping (or crashing). It would start at zero. Then one. Then two. Then five. Then twenty. It tends to go like that - it just clicks at some point and then there's no stopping :)
They'll need a "start" for a while, possibly for a year even given their age. You can help with the "start" by raising the seat some, but of course that makes it harder to stabilize when stopped, so it's a tradeoff. Many kids manage by using the curb, if you're on a very quiet street; the curb lets them have a stabilization point above the ground, so their seat can be higher and still be able to start.
The other piece of advice I'll give is: don't push them beyond what they want to do. 3.5 is very young for this - so it's totally okay if they don't really want to do pedal bike quite yet. They'll get there! My oldest started just before his fifth birthday, because a girl down the street had her first bike, no training wheels, so of course he had to also. My youngest started at four - because his older brother was, and of course he can do anything older brother can. But in both cases we waited until they wanted to - and now they're biking all over creation...