Bungie has apparently been getting asked why the Halo 3 beta period is so relatively short at 3 weeks, and also likewise how come there are just 3 maps in the beta. As you can read below for yourself, it's mainly due to resource allocation - the longer they spend allocating resources to the beta, the less time they have to apply what they learn from it to the final game. As for the maps - they basically aren't all finished yet!
I’ve been asked a few times why the impending Beta is only three short weeks long. Well, obviously the number one reason is that the longer we’re spending resources taking care of the Beta, the less time we’re applying what we’ve learned from it to the final game. And statistically, three weeks is going to seem like a pretty long time when you look at the hours you log on the game.
The reasons you’re getting three maps is similar, but has as much to do with our normal production schedule as anything else. It’s easier to think of why you’re not getting the rest of the maps. They’re not done yet. All are geometry complete, but they’re being polished and tuned for graphics and gameplay ‘til the last minute. You could say that the maps you’re playing are the best balanced and most complete so far.
But they will change slightly, maybe noticeably when the game finally ships in fall. So by all means, learn ‘em well next month, but don’t get used to anything you see in there. Objects may have shifted while in flight.