There is no definitive answer, as what is the best bike for a 3 year old will vary depending on their individual needs and preferences. However, some good beginner bikes for boys and girls in this age range include the RoyalBaby Kids Space Shuttle Bike, which comes with three different wheel sizes to choose from. Alternatively, the Razor A3 Electric Dirt Bike is a great option if your child is more interested in riding dirt bikes than bicycles.