Question: A 3-year-old does not want to play with other kids; instead, he prefers to play alone, even when encouraged. What should a parent do in this situation?
Answer: I think the first question you always want to ask when a child appears not to want to behave in a way that's developmentally in sync with where you expect them to be is why. What's stopping that 3-year-old from wanting to be with other kids? Is this a sudden change -- had the child wanted to be with kids, say six months earlier but not now? Is this something that's always been present, which would again suggest that this is normal temperament?
Once you've investigated all of the possible reasons why, you might try to facilitate play dates for the child but in very controlled circumstances. Some kids, especially temperamentally anxious kids, will do better if they understand that the play date has boundaries around it: It will be 30 minutes or it will be the length of one TV show. Ease the child into it before you decide that there's something very, very wrong with the child.