My dad tried those fruit trees - a 5-way apple, and a 3-way cherry, and I've tried a couple of mixed citrus as well. Either, as Caroline says they die off altogether, or one type that is the most vigorous of all the fruits takes over the whole tree. Pretty soon you have only one type of fruit anyway. I had lemon/orange/grapefruit on one tree and it looked great for the first year. After its first winter, the grapefruit grew tremendously and the other two died right out.
So, yes, I'd agree it's a nice idea but I have serious doubts you'd ever get one to mature with more than one type of fruit still producing.
