What the guys say above is exactly right.  One ton of cooling = 12,000 BTUs.  The model number reveals the tonnage if you can read it before it fades away from the sun.  So like Rick says a 12 would = 1 ton an 18 =1 1/2 ton , 24 = 2 ton, 30 = 2 1/2 ton and so on.

As a rule of thumb you can figure about 400-500 square feet per ton of cooling.  This all varies depending on your heat load on the house.  Heat load calculations can get complicated.  The rule of thumb usually is close enough for a garage!!

 

If the Carrier unit was truly a 3 ton and they did not match the air handler with the right refrigerant control it could have added to the problem..
Glad to hear you're chillin once again!!