Oh, that's easy!! If you wind up on the 1st floor, just politely ask the team member with the clip board that asks how many is in your group that you would like to have car 2, as there is 3 cars in each dome on the bottom with car #2 being the middle car, even if you have to wait a but longer. 99% of the time, they were going to put you there anyway, or just say yes. Only reason why you wouldnt is if that car is down for maintenance that day, especially if only 1 dome is operating.
On floor 2, there are 5 cars to each dome that are numbered 4-8. So ask for car 6 as that is the middle car. Even if you get 7 or 5, still ok. If you get the very end cars, 8 or 4, get ready for the worst view of the ride ever.
Havent been to the 3rd floor yet, but I think I remember being told by a team member that there are 4 cars to each dome on the 3rd floor.