Ever think about a mobile unit on each side? Like hire someone to put a hole in the wall to vent the hot air and have one on each side and have two mobile units running, that should cool off a 3 car garage in no time. Then have a dehumidier in the mix somewhere.
I have two mobile units (one A/C, one dehumidifier.) The garage I work in (my godparents, seems I'm there most of the time, gotta be the garage, no cable modem, no, wait both!) is not insulated but it will get cool with the A/C on in there (it is a 2 car garage.)
Just my 2 cents.