As someone who used to work there, heres how it goes. The three theaters are A, B, and C. Coming up to the split, Theater A is to the left, Theater B is to the right, Theater C is straight ahead.
If you're standing in the Theater looking at the screen, Theater A carriages from left to right are Gate A, Gate B, Gate C.
Theater B and Theater C are set the same way, however opposite from Theater A. Looking at the screens, from left to right, are Gate C, Gate B, Gate A. You can also think of it as the first Gate you encounter, regardless of what Theater you go to, will be Gate A. (Someone said earlier that they are laid out the same to prevent confusion since the CM's move positions so much. However we do move around to every position often, the Theaters are not the exact same. Others, myself included, have definitely walked guests into the Theatre then went the wrong way for a second before realizing which one we're in).
If you are sent to Gate A, Row 1 first, you will be directed all the way down, which will place your seat on the edge of your carriage, right next to the Gate B carriage.
If you are sent to the Gate C, Row 1 and are the first person, you will be sent towards the exit doors, so if you do get Gate C for any Theatre, ask to be last, you'll end up on the last seat, which means you will be right next to Gate B carriage. Hopes this makes sense.
Honestly just wait for B1.