Yeah, if you assume everything else is in perfect condition... then your 3 ton might not be able to hold 70 F. Usually when engineers or contractor do the load calculations to choose your condenser and coils, they assume the residents will run the AC to hold at least 74-75 F. The goal these days is to undersize units... I would still maybe sure you are getting the correct amount of air through your supply registers... adding the return grille from my quick calculations won't give you much.
If you bought one of those houses from a national builder... they will put the same system in all their homes no matter which way the windows face or house is oriented.