Three is a magic number for me.

Two is fun, but not quite enough to have an "I win" button!

Four or five is ok, but the stress levels go through the roof at times, and for me personally that falls under the banner of "doing it for the challenge" rather than purely for fun.

Three is just right

Of course, it depends what you intend to do. I'm very lucky in that I have a wife who plays (and multiboxes!) and a RL friend who lives literally down the road, so it's easy for us to co-ordinate an instance run or something.

On their own, three won't let you do *alot* more than two. The biggest deal would be if all three were shammies, in which case the benefit of 3 stackable / periodic totems over two is dramatic.