In reference to what is the "best" way of doing 3 box.... well the answer is, What will YOU enjoy most....

i tend to stick with simplicity, 3 classes same spec same gear allows for more effective game play. when you add a 2nd class in your make up or even a 3rd/4th you tend to run into micro managing roadblocks..... If you can overcome those roadblocks its possible to be a more effective multiboxer....

i think for a new multiboxer your best bet is shamans or hunters.