In general, I highly recommend the Kettrike Air Navigator. It is a great trike, very well made and will last through several children (if need be). Depending on how coordinated your 3-year old is, a small bike with training wheels may be a good alternative. Or a balance bike - those are very popular right now. Kids learn to balance and ride, and then moving to a real bike is a piece of cake - you can skip the training wheels altogether.
We have a lot of options on our website, and we're happy to answer questions: