This can be a tough one. I started with one blog (about programming) and decided I wanted to focus on investing and personal finance as well. Well, that seemed like a big enough leap from programming that I decided to create a separate blog for it. A month or so later I started up my 3rd blog.
I personally like the idea of them being more focused on a particular topic or two instead of really jumping around a lot of unrelated subjects. I also felt that as the blogs grow, readers would be more interested in a grouping of topics as opposed to an all-in-one blog. I could be wrong though :-)