Click on
http://hvac-talk.com/vbb/showthread....1#post15583161 and read the post again.
If your 3-Ton ran steady at the worst conditions then it was sized right; though it may not have been delivering to & from the rooms near its Rated Performance. They can't size replacement units by un-performance tested existing units; the existing system may need work to improve its performance!
There is NO way you can go to 5-Ton on that duct system & Return Air Filter sizing, etc., don't even consider it; please don't make any equipment decision until the Home Energy Audit & any needed Retro-work is completed.
Please don't get the cart before the horse; You could make a costly mistake that you will regret for the life of the equipment.
Were it me, I would use a separate system for the distant room that is running the load-calc way up. The design conditions show that humidity is probably the biggest problem not temp; therefore, never oversize the A/C, as it needs long runtimes.
Check the duct system & return Air filter area
sizing should be for 300-fpm velocity through a 1" deep fiber glass filter (Manual D, Bet they don't know that); you have to keep the pressure drops low...You make the call, you live with an efficient result or, with a very costly result...