3 will work for almost everything. In fact it'll be easier to list the rides that 3 won't work easily for.
Magic Kingdom:
Adventureland:
everything will work for 3 people. You can fit 3 people in one row of the Magic Carpets
Fronteirland:
everything will work for 3 people. I've seen three people in one row on both Splash and Thunder so it can be done.
Liberty Square:
everything will work for 3 people.
Fantasyland:
Pooh may be tight but it can be done
Dumbo may be too tight
Barnstormer probably won't work (it's tight for me and my 3 year old son)
7 Dwarfs probably won't work. It's tight for me and my 5 year old daughter
Tomorrowland:
Speedway may work for 3 people but it will be tight
Astro Orbiter won't work for 3
Stitch won't work for 3 people (to everyone else: I know you can do Stitch with 3 but I"m trying to save this family the agony that is the Stitch attraction).
Buzz Lightyear won't work for 3 people
Epcot:
everything will work with 3 people.
Test track, Soarin, Nemo, Living with the Land, Figment, Mission Space, 3 Callaberos are all good for 3 people
Spaceship Earth would be tight in one row but you can do 2 kids in front and the adult in the back row
Hollywood Studios:
Rock N Roller Coaster is 2 single seats side by side so you can't sit 3 in a row but you could do kids in front and you and a stranger in the back (they fill in open seats with a Single Rider line)
Toy Story Mania won't work. It's two people next to each other back-to-back to two others. So you could sit behind them but your backs would be facing each other. You wouldn't see them.
Animal Kingdom:
Everything is good except Expedition Everest which has two seats side by side. So you'd have to sit in a row directly behind them. And the Triceratops Spin which is the same set up as the Magic Carpets (so two in front and one in back row).
