Do kids personalities change over time? My son is suddenly shy at age 3?

Yes, my daughter has definitely gone through tons of different personalities changes/phases. She's usually a very out going, loud, out-spoken little girl - but sometimes for a little while she'll go through a different phase. Right around her third birthday she went through an extremely clingy phases. She used to walk right up to people and act as though she knew you for years, but then she got very shy and would only want to hang by my side the whole time. She eventually grew out of it. She'll be four in August and is no longer in that shy phase. ;)