So I'm thinking about getting this game, and I'm just curious that would you need any of the discs to run in the 360 when the whole game is installed on your hdd?

The thing to do is install discs 1 and 2. 3 is unnecessary and may actually cause problems. However, even though they're installed, you need them in the tray to spin up and verify they're legit. So when it's time to pop in disc 2 you'll have to actually get up and do that. Really not a big deal. The install is mainly to stop texture pop-in but in my experience the difference between installed and not is minimal at best.

same as what boomslang said but you'll need atleast 23 gig of HDD space

I only have a 20gb hdd.. with just half of it in free space, could I install one disc and play from that and then uninistall it afterwards?

Install disc 1 until ur done with it then uninstall and install disc 2. I dont have this problem i have a 250GB and soon i may just buy the 320 mw3 edition.

You can install both discs and still have room. I installed both discs and it was only about 15.8 gigs. And that's including both installs, Rage video downloads, save game info and other stuff.

I installed the first disk, then decided to install the second, however I don't see the second disk install in my 360, but I did see the install bar. When I went to my games on my 360 I do see disk one, but not disk 2. Can you only install one disk at a time untill it prompts you to use disk 2?