...until it turns out two of the three founders tend to agree on most issues, and the third is constantly overruled, feels left out and angrily quits. I don't think it really makes that much of a difference: there are pros and cons to both 2 and 3 founders.

I heard something similar, but with a slight twist - when any decision is to be made, make sure you have a prime number of deciders, otherwise there is the possibility of deadlock. I think that's from the founder of Sony? He had a lot of cool ideas like that.

Not too many companies have 9 founders, though, so I guess your version works well enough for startups.