I have a smallish orchard of about 3/4 acre. I have my trees spaced 15 feet apart, and rows at 20 feet. my trees are less then 10 years old, but seem to have plenty of room so far. I did 15 feet, because when I go to commercial orchards, they have the trees at like 8 feet, and the branches are all touching. I figured if they can do it to cram as many trees as possible in the area, and keep a increase of apples, so can I. I think with your normal spacing of 20 feet, will give you maximum amount of apples per tree, but not necessarily per square foot of land. At least that was what I was told from a worker at a commercial orchard.
I could see the benefit to doing 3 trees in one small area, if you didnt have the room to separate them. You wouldnt get anywhere near the same amount of apples you would if you planted them apart, but if you didnt have the room, and wanted 3 different kind of apples, it would work, you would just get less apples of each kind.