I have one of the many 3-year-olds on here.
DS turned three in November. He hasn't been tested or evaluated at all, so we have nothing definitive, to the extent that any of those are "definitive" for preschoolers anyways. But his abilities with reading, writing, and numbers are pretty amazing -- he catches professionals by surprise (e.g. he read all of "One Fish Two Fish" aloud to a very surprised pediatrician yesterday, and his preschool teachers are watching his skill acquisition with interest). He also memorizes songs, books, sequences, etc. really easily. So, yeah, probably gifted. Don't know more than that yet.
Strangers couldn't tell his giftedness from interacting with him, though! He's taciturn -- he doesn't say much about what's going on in his head, and his spoken vocabulary is fairly small. Also, he's kind of quirky in a lot of ways. He retreats into a social shell when in a large crowd, he hates crowd noises and loud music, and he tends to self-stimulate with repetitive motions or counting out loud. (We've learned that these are his coping mechanisms, and we respect them as such.)