The "thrill" rides (use the term loosely) seem to have definitive seats, mostly 2 together (Splash, BTMRR, Buzz, Pirates, Rockin Rollercoaster, Dinosaur, etc.). Space is single seats all in a line. The rides w/ bench seating should work w/ 3 - Peter Pan, Haunted Mansion, Spaceship Earth, Maelstrom, Ellen's energy ride (what's the name?) Great Movie Ride, Safari, Jungle Cruise, etc. Test Track is 3 seats together, Mission Space 4 "together", Kali 6 in a boat.... and on....
If you have specific rides in mind, why don't you post them and I'm sure someone can give you the info. I had the same question when we went - we let DS pick who he rode with and where he rode, which worked out well.
Have fun!
Beth