Three20 was Joe Hewitt's baby when he was working on Facebook iOS stuff. After he stopped that, it seems like the project essentially went to seed. My understanding is that the 4.0 rewrite moved over to Nimbus (http://nimbuskit.info).
Three20 is hard to use, and is, for the most part, an all or nothing affair. You either use it throughout your whole app or not at all.
Three20 was in vogue (if it ever was) about three to four years ago. It was an ugly abstraction because it was very web-development centric. Most good developers I know have long abandoned Three20.
Really, you're not a real iOS programmer until you write your data abstraction yourself... and rewrite it two or three times based on lessons learned.
Three20 is hard to use, and is, for the most part, an all or nothing affair. You either use it throughout your whole app or not at all.