Adding a third A/C unit is no more difficult that adding a second A/C unit to a TT, 5-er, or Toy Hauler. I added a second A/C unit to my TT and I have never regretted it. Being as you have a vent in the garage area probably 1/3 of your work is done. I did not attempt to use a ducted A/C unit, I was mainly trying to cool the bedroom on our 31ft. TT. For your TH with the vent in the garage it would be easy to add an A/C unit with the air distribution box (ADB) with the adjustable vents built in. IF there is no blank plate on the ceiling and/or a blank plate on the wall for a t'stat then you will have to provide electrical power to the unit and with the non-ducted ADB the t'stat will be built into the ADB. With a TH I would run new wiring for the A/C unit through plastic wire way across the ceiling, down the wall to a point where I wanted the wiring to pass through the wall. I added a marine grade, a "Marinco" brand, that goes through the wall and has a male plug on the exterior with a rubber snap on weather cover. I use a 10 ga. electrical extension cord which I plug into the 120vAC duplex receptacle on the power pedestal at the CG and the female end goes onto the male plug on the TT. I terminated the wiring inside a 6x6 electrical box within my front storage area and added within that box a marine grade 20 amp push-button circuit breaker. This set-up has worked very nicely for three years now. I would bet that a dealer would charge you in excess of $1000 to add a third A/C unit including the unit, all wiring and labor. OH-getting the new A/C unit on top of the TH is the fun part. Leave the new A/C unit in the box, set an extension ladder to reach over the top of the TH (secure the feet of the ladder) and use a rope to pull the box up. Call a few friends and tell them you have a lot of extra beer on ice and they should come and see. I did a little different, I set two bottles of good wine on top of the TT, showed her that the A/C box had to be up there before she could have the wine. Effortless on my part.
2015 FR Freedom Express 310BHDS pushing a 2014 GMC Sierra CC