I've had 2 and three person partnerships... One person tends to grab the thought-leadership and the others tend to follow (tho hopefully not so much that they don't speak up with great ideas).
I would advise having a buy-sell agreement that allows a graceful exit if one person wants out, but otherwise I'd agree that partner selection is 50% of the battle (and letting go of your own ego is the other 50%).
For buy sell agreements, the best I've seen, simply stated, is this:
At any time, partner A can make an offer to buy Partner B's stake for Value X (whatever they want to offer). The rub is that Partner B can decide to reverse the offer (buying Partner A's stake for Value X) and Partner A is obliged to sell.
Hopefully you'll never need such an agreement, but it sucks to feel trapped in a business (or trapped with a partner who has turned into a lunatic or is no longer pulling his weight).
So far, every partnership I've had has been an even one, and I've never had to exercise an buy-sell clause like that (knock on wood!).