Issues associated with PDD may be contributing, and as you said, they may be feeding off each other. I agree with others that they are at prime developmental stages and just be very very very consistent with their routine and them not leaving the room, but they must go back to sleep.
Things that may help are keeping the room pleasantly cool, white noise, enough DARKNESS (no blue or white lights, if you need some small amount of light, look into orange), if your 3 year-old wants water at night, have a sippy cup with some water available for her to drink by herself. Here are other ideas (some are for older kids), but always ask your pediatrician: http://itsnotmental.blogspot.com/2009/01/sleep-timing-of-...
Make sure gut health is fine, as this is often and issue in PDD/autistic-spectrum children. Oh, and then there are allergies to consider- both environmental and to foods http://itsnotmental.blogspot.com/2011/05/allergies-asthma... but, I am coming from an experience with one of my children having some MAJOR, persistent issues with sleep and that contributing to a host of issues (although it is hard to say what caused what).