What ton ac unit needed for a 1400 square foot three bedroom apartment?

You usually go by the theory 500 sq. ft per ton of air, but it also depends on how well the structure is insulated. If it is not insulated well then you will need to go with 400 sq. ft per ton of air. If the structure is insulated very well then you can go with the theory of 600 sq. ft per ton of air.... So, for 1400 sq. ft. you would probably go with a 3 ton unit.