Three years old is really young to have all of his neurology together to ride a bike w/ no training wheels.
There's motor planning involved in pedaling, motor planning involved in steering, balance (from the vestibular system -- I'm not sure when that system is ready to ride a bike), the motor planning involved in stopping the bike, plus all of the attention shifting, gaze shifting, shared attention (joint attention) required to keep from running into someone or something and to keep out of the street. There's probably some hand/eye and foot/eye coordination needed, too.
He may be tall, but developmentally, he may not be ready.