Depending on the age and type of box you may be able to attach another device box to it. The old box needs to be an expandable type with screws holding the sides on. The dimensions for the screw placements have moved around over the years, so it could be hard to find a compatible box to attach. You will need to make a larger opening and do some wall repair to get the new gang in and attached. I would remove the old box and move the cables into a replacement 3-gang old work box. If done very carefully the hole in the wall for the new box will be the correct size and no wall repair will be required.

Devices all count the same for fill (despite dimmers being larger than switches). Just because it is installed does not mean it is correct. Having to dig into poured concrete for box depth is not the worst thing in the world. Drill some holes and chip out between them. The EMT makes rework harder, but not impossible (slip connectors are your friend). Fit can also be affected by the brand and style of GFCI device. Some are smaller and have better terminal locations for minimum allowed box size. A device with side screws can touch the back of the box you just need to dress how the wires come off the screws. Pinching wires between the device and the box (especially a metal box) can cause shorts, sometimes nasty ones that are hard to diagnose since they are not solid enough shorts to trip the breaker all the time. The buzzing and smell may be the only clues.

What you have is a multi-wire branch circuit (MWBC). Properly installed, they are perfectly acceptable and legal. The key point is that the two hots must be on opposite legs in the panel. Because of the way A/C works, two legs are out of phase by 180 degrees so the neutral carries the difference in current between the two legs. It's biggest advantage is cost savings because you save one [neutral] wire for every two circuits. Frankly, I'm not overly fond of them in residential setting because people don't understand what's going on and there's the potential to get zapped. It's critical to shut off both breakers when working on a MWBC. The rules say that the breakers should be tied so that both are turned off at the same time but that doesn't always happen. Let's say that you have a room where a wall receptacle and overhead light are opposite legs of that MWBC and you decide to replace the receptacle. Unaware it's a MWBC and because the idiot who installed it didn't tie the breakers, you turn off the breaker for the receptacle and test it. It's dead and you start pulling things apart. Because you want to see better you flip on that overhead light. The receptacle will still test dead but as you're handling the neutral you hand brushes up against the grounded metal box. You get zapped. It's not likely to kill you but it's going to hurt. Change the scenario slightly where there's a table lamp plugged into the wall receptacle and you're up on a ladder working on the ceiling fixture. Get zapped slightly, naturally jerk back, loose your balance, and fall off the ladder. So make sure those breakers are tied.

I looked online a little and it appears you'll probably have to go with a variable speed slider-type fan control. The problem is that from what I found the largest you'll probably find without getting into commercial style controls is 5 amps. If the cooling fins won't work in the box and you have to break some off you have to start derating the maximum load. Breaking off two fins may drop you to 3 amps. Next is the derating for 2 or 3 gang boxes for heat dissipation purposes and you may end up being down to 1.5 amps. So, if the controller will fit in the box with it's full heatsink, you may be looking at 3 amps max. You'll then need to find fans that don't exceed 1.5 amps each. I think you may have some work cut out in coming up with right combination. But if you get stuck, you may want to consider remote control fans, or just add-on controllers for non-remote fans, and just put a toggle switch in the box. Now I got to get back to toasting marshmallows over the campfire and the crickets chirping. :)