Well, if you (or someone else) bumps a three footer, KABOOM! down it goes. I would guess most manufacturers don't want to have that happen.
Think if you had a couple of items stacked near the top of a rack, and someone slips and uses the equipment for a support KABOOM down they go ripping up your cables, the equipment gets dented, the persons foot is injured and they sue you... I can see this happening way too often with three footers. (I own a Rega TT three feet, and HAVE banged it from pressing down on the 'wrong' edge.)
So if your stuff is in a special room no one else is allowed in...
Then, the most important, who has proved three are better than four? With three you have two corners unsupported, and ready to vibrate even MORE than four feet. Most four foot items have at least a partial cushion to allow compensation for a slightly warped surface. The only problem with four feet I can see is if you use tip toes or other hard cones and do not bother to 'test' the tension on the feet so they are balanced. (I have used slips of paper to adjust the foot tension using tip toes) and adjusted the four spikes on some speaker stands so they are solidly in the floor supported equally on all four spikes.
Then some companies have FIVE feet, four in the corner and one in the center, and I do that myself by using a fifth foot I add when the item is properly located in the rack. And usually stiffen the top of the unit with items too.
So, I could never see buying a heavy chunk of equipment with three feet. (I havve a Sony SCD777ES on the TOP of my tall rack, and would have nightmares of chopping off a (human) foot if it had ony three feet.. (that sucker is HEAVY!! i can barely lift it up there!)
Sure some offer three foot items, but it is as much a 'trick' of marketing as any real purpose. IMO.
But for those who want them, all one usually has to do is unscrew the stock feet and use a threesome aftermarket set.
