After rereading your post, it's quite apparent that you actually don't know what you're talking about. Saying the Halo multiplayers are the same is just..well, stupid. Couldn't be more wrong.
They don't mimic each other.
Yet, they are very similar. What is new? New guns? Maps? That's it!
Engine is the same, as most of the guns are. Some of the guns were changed, yes (such as the needler from Halo 2 to Halo 3).
I thought there was a decent change from Halo 2 to Halo 3. However, I see Halo 3, ODST, and Reach as virtually the same thing.
EDIT-
I actually did have Halo 3. I sold it to my best friend 2 weeks ago. Yet, I did not go out and buy ODST/Reach. I rented them, yes. Buying them wasn't worth it because it was the same thing.