Who in their right mind would use 3 at the same time? At that point all the pokemon that are holding it are basically getting no exp.
Sure you can EV train 3 pokemon at the same time. That only has limited usefulness because it would just be faster to EV train a single pokemon using the held item for that EV. --- http://www.youtube.com/watch?v=2WZsZReahfg Mewtwo has got his throne back! BST 780! Take that Arceus!
Well, I used two at a time when level grinding to fill a pokedex, but I'll admit, 3 was kinda crazy. Not that I minded, since, for some reason, if you take all of one item out of your back, the next time you put it back, it will be at the bottom, rather than where you had organized it before. I just left the third one in there as a "bookmark" of sorts.