I used to think it was great to take on a ton of things at the same time. Now, I just look to simplify and actually make progress. I have a strip of black electrical tape on the door and I focus on just a couple things each day that are above the line and move everything else below the line that isn't helping me move the business forward.
I would have to say this all depends on the overall objectives of the company. If you've got a life style start up then one person is enough. If you've got lofty goals and you're looking to grow the company, and even raise money for that startup, then you will want more than one founder.
Finding one other person is hard enough -- trying to find three seems really challenging. There's ways to make decisions and break tie-breakers without having a third co-founder.
I believe that it comes down to expertise. Its very likely that you're not going to have domain expertise in everything, so you'll have to find some other co-founder that does. I'd think that you'd want to add more co-founders to ensure you have the domain expertise covered.
There's a blog post on our website that discusses "Why Three Founders in Better than Two". Might want to check that out for ideas on why it is better to have more than two.