How old are the members of your group?
Many more rides fit 3 than you think. I was there most recently in April with DS14 and DD10. We did not do DHS this trip but went on almost everything at the other 3 parks and split up at the following:
Animal Kingdom:
Expedition Everest - this is only 2 across and they will usually pull someone from the single rider line to fill the empty space
Epcot: nothing here requires you to sit alone; unless you count that on Spaceship Earth we sat 2 and 1 front and back in the same car
(3 people fit fine in a Nemo clamshell)
Magic Kingdom:
Astro Orbiter - 2 young children would fit with one adult if that is your scenario, the odd person out is not with anyone else but gets their own rocket; same with Tomorrowland Speedway
Dumbo - I've seen people with 3, but we've always split up so that the kids can fly their own so I can't speak from experience
Buzz Lightyear - fits 3, but we split up so everyone gets a laser; we have never been put with anyone else
Barnstormer - I don't like this one so the kids just ride without me.
Some that fit 3 across fine that you might not realize: Haunted Mansion, Peter Pan, Winnie the Pooh, Splash Mountain, Big Thunder (Splash was a little tight now that both kids are adult height (plus I had just bought souvenirs) so we chose to split up on BTMRR; there was a group of college aged men all together on it though)
Just let them know the size of your party and the CM's will instruct you. They often asked us if we wanted to ride 3 together or 2 and 1 since I was traveling with older children who could ride alone. It's much easier than you might think.
