(adsbygoogle = window.adsbygoogle || []).push({}); Is it possible that I can overwrite a mw3 patch.mp.ff with my custom patch.mp.ff

no you need a jailbreak to put modded patch on plus you can't run a modded patch on mw3 without a modded Eboot.bin witch you might aswell RTE or RTM and you got wrong section try looking in call of duty classic for mw3 section :p

Yes all that ^

Plus there are no MW3 Patch Editors out. You would need to make your own, and also even if you do get to change a patch it wouldn't work online. It'd throw you an error.