If you are looking for a rationale why 3 is a good number, then I'd have to say: 1) It shows you are not a one trick pony. This is essential if you are applyin

If you are looking for a rationale why 3 is a good number, then I'd have to say: 1) It shows you are not a one trick pony. This is essential if you are applying for faculty positions. 2) It is the number employers expect to see. This is really determined by the number of papers had by your competition. In this regard, the reason the number is 3 is because other people have around 3 papers. This said, no one stamps a PhD on you when you get 3 papers. I actually got 3 papers pretty early in my PhD. I spent an extra year working with a research lab on an unrelated project. I found my PhD committee (full of some famous people) was not impressed by my papers. However, when I had some decent job offers, I felt I grew up in their eyes. So it is really about when you grow up, which doesn't happen by magic when you cross a certain number :-)