Maybe all three could rotate, keeping two games a month? Or are there a lot of people that still have only vanilla?
I like Warlords more than vanilla, but I think it would be a shame to drop either one. I'm a big fan of the improved AI - it has its quirks, but overall the smarter AI is more fun to play against, even if it's harder - and the other aspects of Warlords have grown on me as I've played it more. The UBs add a whole lot more to the game than was apparent when Warlords came out and the expansion adds other complexities and features that are subtle, but strong improvements.
However, since vanilla is different enough to stand on its own and is the original, it doesn't make sense not to play it.
How Beyond the Sword is, of course, affects how things are done.