Very strange guys.After I tried everything in my router,I got my old one whick was already reseted, connected to to internet and now its working like a charm with nat open by default.I tried switching routers and yes, my new one cant play MW3 although I can play all online games.

Guys I think I found the solution.Login into your router config page and change UPnP to the opposite setting (if it is enabled then disable it,if it is disabled then enable it).My old router works with UPnP enabled and my new one with UPnP disabled.If you manage to login into multiplayer and you find strict or moderate NAT,then do the "famous" port forwarding on these ports (check that NAT is enabled first it router page):         TCP:3074,27014:2705 UDP:3074,27000:27015,27015:27030 .Post back if it helped.