i had the same problem as you, you need to play more defensively in mw3. And also, learn the maps, spawn points, and how to handle the recoil of your guns. Plus the fact that Infinity ward and Treyarch make different games and engines might add to that. Lastly, watch A HELL OF A LOT of MW3 videos on youtube, from ppl like Xjawz, TheMarkOfJ, WoodysGamertag, etc can also help you to learn the maps and do better. BTW,
If you want to play PS3 or Xbox, my PS3 is: SharpShootah_B i hold about a 1.56 KD in MW3 and a 2.14 KD in Black ops. Occasionally, i play Fifa 11 and 12 but mostly COD and other FPS games. My Youtube is: skyvoc2000 so just send me a PM of your PSN/GT and i'll add you get back to you. Really i'm looking for a party/clan to play with most times i come online in MW3 (mostly on ps3)so if your looking for someone to join your Clan/PSN party, or someone to have fun playing with and have a great time just send me a message cas im your guy
(BTW i will be getting my xbox bakc in a few months after it has been fixed so keep that in mind)