Many of the rides you can sit with two adults and one child, but depending on how big everyone is you might be more comfortable switching who rides with whom. In Fantasyland all of the rides you can ride together, with Dumbo being the toughest to fit I would imagine. In Tomorrowland Stitch has individual seats but you are all together. Buzz, COP, and TTA, you can all ride together. Goofy's Barnstormer in Toontown I don't think you would all fit. HM yes. BTMR would be a tight fit. Splash yes but probably a tight fit. Pirates yes, Jungle cruise yes.
You will be seated right behind each other if you decide to ride one adult and one child with the other adult in the next row. That would probably be the best solution as I have ridden all of the rides three in a seat with myself and my two kids, but as they got bigger it got more uncomfortable.