Maybe I'm nuts, but I don't see three people in a Splash Mountain seat. Three in the same log, sure, but not in the seat.
An average-sized adult and 2 kids little enough to fight about riding with Mom should fit fine on HM.
Yes, again on Small World, but No on Buzz. Space Mountain is one-per-seat, but you ride together.
Test Track will fit 3 in a row, so will Mission Space and Soarin'.
Tower of Terror, yes, Rock and Roller Coaster - I don't think so.