When My son was 3 we got a bike for him. It was a small one, but it would last two years before he needed a bigger one. It had training wheels on it and we lived in the country with a long dirt road. He got on the bike and within a minute he was spinning his pedals and wheels but not going anywhere. Holes in the driveway .....hahaha. So we took off the training wheels and he never needed them put back on. He learned balance on his own.
Just my two cent of experience.