Kids can be pretty divided about growing up. My grandson did this for awhile, also around three. I asked him once if he wished he were still a baby. "Yep."
Why, what does a baby get that a bigger boy doesn't get? "Cuddles, snuggles. More help. Not so many rules. More smiles." (I read into his reply, "more encouragement and positive feedback" from adult.)
So I suggested we play for awhile that he's a baby again. During play, he'd lapse into big-boy options, and I'd have to remind him babies don't get to do that. After 5-10 minutes, the game got old and he wanted to be a bigger boy again.
I asked him to use baby talk only when he wanted to play that game, and for the most part, he did. We "played baby" almostevery week when I visited him for a few months, and now, a year and a half later, he still occasionally asks for the game. He longs to get his attention/affection cup filled in ways he no longer quite believes is appropriate.
I also make a point of giving him lots of snuggles and encouragement. Baby talk seldom emerges outside of the baby game if he's getting what he needs.