I know I'm catching this one late, however, there is a side to this that seems to be missing in the answers I see here.
While this behavior is asolutely normal in a three year old, and Skippy is right that reasoning with a three year old has little chance of getting you very far, there are a few things you can say while doling out any chosen consequence for hitting or for throwing things. "I see you are angry" or "I see you are frustrated" are two of the most important. At this age, very few kids have a full ability to identify their feelings, let alone communicate them verbally so they go for the physical or crying etc. By "putting the words in their mouths" you do two things: teach how to communicate their emotion in an appropriate way, and you let them know you understand. As you move your child, to a safer location, I highly suggest saying, "I see you are angry, but throwing things is not okay even when you are angry." Then don't say anything else - it keeps it simple.
Most importantly though, only dealing with the symptom (the behavior itself) doesn't fully address the issue. While his behavior is normal - even expected to some degree - the problem also arises from the fact that your son is angry. If this is new behavior and the baby is also new, I'd bet the two things are related.
All three year olds get frustrated and all three year olds have tantrums in some form or another. Babies mean a lot of changes in your household and your son probably doesn't really understand why those changes have happened. It is fairly common for three year old kids to be resentful of a new younger sibling because they had no say in the matter. Developmentally, it is as if a baby dropped in and decided to stay and took over. If you want to address this problem from all angles, you need to address this too(if you haven't already).
I suggest getting some books about babies and being a big brother or sister you can read with your child, really making sure to make time for each of you to spend alone with your three year old, and giving your three year old some jobs to do (when he isn't angry) that make him feel like a big helper in caring for the baby. These jobs can be as simple as, Its your job to hand daddy the bottle when a bottle is being prepared, or handing the diaper up during diaper changes. This question might help with this part of your challenge.