Why Trane? It's a good unit. Never had to service one (not saying they're bulletproof) but it's a quality unit and not just a name. You can get parts easily. I know alot of techs around me complain about Yorks and the availabilty of parts or equipment. It's a good unit though. Everyone has they're little preferences and ones they install more than others. Me, I've installed alot of Comfortmakers. Only time I had to service one is when my in laws flooded the unit TWICE. Utility room was directly above it. Anyhow, it survived the first flood but blew the fuel valve second time. I installed a Comfortmaker furnace in my house. Good furnace, a little loud, but whatever.
I have no clue how your house is set up or anything so I can't say 3 ton is right or wrong and I won't. What I can say is that you need that calc done, and you should have it prior to accepting the bids JMHO. If you go with a unit that's too big you lose your eff rating and it will short cycle, meaning it will only run for a short period of time when it calls for cooling. You may have your stat in a bad place, who knows it's all speculation. A quality contractor will take in all the info of the entire system and will tell you problems that need to big fixed and will tell you PRIOR to accepting the bid. If something isn't on the bid that should be changed, he can and will charge you extra after they get there. A pretty bid sheet doesn't make a good contractor. His knowledge and willingness to share his info prior to acceptance makes him a good contractor.