When my DD was 3, and in preschool, I asked her teacher how she was doing socially. The teacher said DD generally preferred to play by herself, and that she was able to say "No, thank you; I want to play by myself" without hurting any feelings when she was asked by others to play. And that she was asked fairly frequently to play, and was always welcome in the groups she wanted to play with, when she was the one who initiated playing together. And that there was a group of kids who preferred playing in the areas DD played in, because she let them interact with her as much or as little as they wanted, whereas some of the more-social kids would insist that the kids in their area play with them.
FWIW, she became much more interested in other people (and in fitting in socially) about the time first grade started. Honestly, it was easier for me when she didn't care what anyone else thought - we did a complete wardrobe change mid-year, among other things.
