You mentioned your dad would be using a wheel chair and not a
scooter.
Since he had a stroke, I'm not going to assume, he self propels the wheel chair or someone pushes the wheel chair.
If its the latter he needs someone to push the chair.
Maybe he would want to rent a scooter. The reason being, you
have lots of kids to keep an eye out for at such a busy place, with
lots of attractions. Having an adult pushing him around, kind of makes it more difficult for that person, and when they are watching their kids too at
disney world.
Plus, if someone is pushing him the whole time, thats is a lot of work.
The whole 3 wheel chair /
ecv at one time is more difficult for bus transportation.
On the other hand, if your dad uses a wheel chair. Is it a regular wheel chair that can be folded up, or does it have special extra on it, which don't make it easy fold. The reason I ask . If you have 2 ecv's and a wheel chair.
If the wheel chair is foldable and your dad can safely get into a bus seat. Someone in your party could hold the wheel chair off to the side, and possibly allow all 3 to fit on the bus. On some types of busses, there are small cubby hole spaces, where you can slide some of the folded wheel chair in, and hold it upright. An adult would have to hold the wheel chair so it doens't tip.
Its not the easiest solution, but could help when using disney busses.
Good Luck