Well, while a 3-year-old might be a Mozart-level prodigy, most experts recommend waiting until they're a bit older before starting violin lessons. At this age, they're more likely to use the violin as a toy, plucking the strings randomly, which isn't exactly conducive to learning to play Vivaldi's Four Seasons.
But that doesn't mean you can't start fostering their musical interests at a young age! Singing and playing simple tunes together can be a fun way to introduce them to the world of music. And who knows, maybe they'll be playing Paganini by the time they're five. Or maybe they'll just enjoy making sounds with the bow. Either way, it's all good fun!