The easiest solution is to pull the switches, connect the wiring, using the space in the box as your junction box, and put on a 3-gang, 2-blank cover plate. The added bonus is that if any future person (including future you) needs a location for a switch or extra outlet, they've got it with almost zero work.
If that's not an acceptable option (and I understand that it may not be), the hardest part of downsizing the hole is taping the drywall and getting the joints smooth. So from there, it probably doesn't matter a whole lot what approach you use.
The most important item is to ensure that you do not exceed the box fill requirements when connecting all your wiring in the new single-gang box. There are questions here about calculating fill, and you could ask a whole new one if you're not sure. If you've got too much fill, you'd need a 2-gang box, and if that's the case, it makes leaving the 3-gang look even better.
To actually replace it, I see a couple of options, depending on what the box is made of.
Option 1 - metal box
- You'd most likely have to open up more drywall to be able to get the nails/screws out.
- Open it from stud to stud so you've got something to attach the drywall to on both sides to make the repair easier.
- Remove the old box
- Install a replacement new-work box
- Install a piece of 1x2 next to the new-work box screwed into the drywall above & below the opening
- Install a drywall patch from the 1x2 to the exposed stud, screwing it into wood on both ends
- Tape, mud, sand, paint
Option 2 - plastic box
- Remove the fixtures
- Using an old screwdriver and hammer or extremely careful application of an oscillating cutter, destroy the old box in place.
- I've done this - it is possible. You need to use great care to ensure you DO NOT cut or nick the sheath on the wiring.
- Remove the pieces as you break them off
- Use the oscillating cutter or a Sawsall™-type saw to cut nails holding the old box to the stud.
- Install an old-work plastic box next to the stud
- Put a piece of 1x2 behind the existing drywall, screw it in above and below the opening
- Screw in a patch of drywall to the 1x2
- Tape, mud, sand, paint
Option 3 - plastic or metal box
- Cut a large enough opening in the drywall to get a hammer above and below the box to pry out the nails holding the box to the stud
- Cut the opening wide enough to span from the near stud to the next stud
- Install a new-work single-gang box
- Install a drywall patch, screwing it into the stud above and below the new box and to the next stud over
- Tape, mud, sand, paint
Option 4 - metal box with conduit
- Depending on where the conduit enters the box, you may have to replace bent conduit with straight conduit, but otherwise, it's like Option 3, but with more drywall removal/replacement
Again, the hardest part of all of these is the last step: Tape, mud, sand, paint. Once you decide on an action that involves opening the wall from stud-to-stud, you don't have significantly more work to do in this finish step, so it probably doesn't matter which one you choose.
The key item is to ensure that you do not over fill your new box with wiring - something that's more likely if you've got #12 wire instead of #14.