Well, not all founders are Threes. It would be interesting to type everyone in a YC class (or several) and see what the distribution was. I'd certainly expect some Eights and a smaller number of Sixes. And Fives will of course be well represented among the technical founders.
(Interesting to see someone plugging the Enneagram on HN. Doesn't look like anyone else has picked up on it, though.)
I am very pleased to see other HN readers mention the enneagram. A good friend told me about it a couple of years back and I can honestly say it has helped me better understand myself and others. I see lots of 3's in startups, along with some 1's and 7's. I agree that 5's/8's seem to make up the majority of engineers I've met.
"...I can honestly say it has helped me better understand myself and others"
And that I think is the value of these typologies.
Reading the Wikipedia page about the enneagram, I see a system of nine types with three sub-types, various connections between the types, 'wings' and 'drives'.
Am I being cynical if I think that system considered in the abstract is sufficiently rich to support almost any interpretation?
There is a lot of meat to the Enneagram. If you want to understand yourself better, it's a great thing to study -- and once you see how it applies to you, you'll see there really is something to it.
(Interesting to see someone plugging the Enneagram on HN. Doesn't look like anyone else has picked up on it, though.)