Hello Gabriel,
Thanks for stopping by planDisney with your question. Fall is a fantastic time to visit Walt Disney World (then again, every time of year is fun). It sounds like you will be traveling with two very special people and will make memories to last a lifetime!
Many attractions in all 4 theme parks will comfortably fit 3 adults, either side by side, or allow 2 adults to sit in one car while the other person in your party is seated directly behind or in front, like
Splash Mountain in Magic Kingdom or
Spaceship Earth in EPCOT. There are a couple of exceptions. I would say the
Haunted Mansion Doom Buggies are much more comfortable with two adults. Although it's not impossible to squeeze 3 people in one car, I'm certain you'll be more comfortable if you split up and one person in your party rides solo.
Other attractions with similar seating arrangements include
Under the Sea - Journey of the Little Mermaid,
Peter Pan's Flight,
Buzz Lightyear's Space Ranger Spin in Magic Kingdom.
The Seas with Nemo & Friends has a similar seating; however, I feel like the clam shells in this ride are a bit bigger, so 3 adults may be okay. You can decide once you see the ride vehicle. Likewise, the cars in
Toy Story Mania! seat four, but they are back to back, so someone will be riding solo in that attraction as well.
Hope this helps. Have a magical vacation!
Diane