As with all things at Disney World, there are ride cars of all types, some that accommodate what you are looking to do, and others that certainly will not.
With the growth of interactive rides like
Buzz Lightyear's Space Ranger Spin and the
Toy Story Midway Mania, you will find that more of the newer rides really are for two people, even though their bench seating would allow for three. The reason is that with these attractions you play a role in the fun, zapping targets with your 'guns', which come two to a car. That means someone on your 3-person 'team' won't have a gun.
Nobody wants the middle seat on the long family vacation, and that same rule applies here!
That being said, the 16 month old will probably be glad just to share a gun, so you are probably OK. But if you want to check on more than just these rides, you will find a photo catalog of ride cars at
AllEars.Net. Just look in the Theme Park section of the site, and you will find pictures that will help you judge for yourself!