Can I use the game in the steam folder,or I must copy the game to another folder??????????? :?:

And I also buy DLC1 and DLC2

Yes you need to copy your MW3 folder to another place.

No you dont need to buy the DLC to play them here.

You are perfectly fine with adding the IW5M patch to your steam folder. But just as a cautionary step, make a copy of your game folder and add the patch there.

 

You can download all the DLCs from here

Actually, IW5M replaces many fastfiles. So, you better copy your game folder to modify it with IW5M