From this week's "20 Answers and 1 Question" at Newsarama with Dan DiDio:
17. What’s the status of Legion of 3 Worlds?
DD: We decided that, even though it’s connected to Final Crisis, the story’s events take place in the future, and therefore it didn’t have to run lock-step in conjunction with the main series. Legion of 3 Worlds is essential to the DC Universe and tied to Final Crisis, but because stories take place in a future timeline, it doesn’t need to come out in the same way that Revelations did.