At 400 cfm per ton, 1,200 cfm is needed at a minimum. But for returns, it is always recomended to go larger. I use an old formula which is simple and always works for me.
Take the total system required cfm X .36, then add that to the total cfm.
Example= 1,200 cfm X .36= 432, 432 + 1,200= 1632 cfm.
An 18" round duct, or three 12" round ducts will work. Then the return grill area in the cieling or wall should be- 14 X 14, 12 X 16, ect for the sake of noise the bigger the better.
The first number in the grill size is the width, the second is the hiegth- so a 14 X 20 would be 14" wide and 20" tall- the stamps in the grill running the 14" width.

thank you! that is exactly what I needed to know. One more question, if you don't mind. This house is being finished in stages. When we move in, one of the 3 cold air returns will not be in use....it will be covered over since that area of the house is unfinished. It may be 9 months or so before all 3 ducts are in use. To plan for this, should I install two 15" ducts in the area of the house that will be in use? I can install a 12" or even maybe a 10" in the other area (the master bedroom) and cover it with plastic until I am ready for it.

The sizes I gave you are the minimum. You can run 15" on all three- even 20" if you wanted. A larger duct won't hurt anything but the pocketbook.
If you want to use just two temporarily, you can run two 14" with no problems.
Ducting usually comes in even sizes, I've never seen a 15" round duct.? I've also noticed that register sizes are different up north than they are here in the south- we use sizes such as 12 X 6, 10 x 6, and so on- I notice up there you use 12 x 4, 10 x 4 and so on- strange.

I am not sure what size round pipes are available...someone told me to get 15" ones. I never actually shopped for them yet. I will go with two 14" returns in the main part of the house and at lease a 12" one for the bedroom then. That should work great. Thanks for all your assistance. This should be an interesting project! LOL!