This relates to the PC version, but I'm sure it could effect the consoles too.
Modern Warfare 3 has the bad luck of shipping next to Battlefield 3, a game that shows off a brand-new engine to great effect, especially on high-end PCs. In contrast, Modern Warfare 3 runs on Infinity Ward's proprietary engine, and that beast began its life as a heavily modified version of the Quake 3 engine. It has been improved and modified since its debut in Modern Warfare 2, but it's still old technology. Those behind the game get a little sensitive when you criticize the use of the older tech.
I can't remember if the PS3 has component connections, but component can carry HD signals. If you have the necessary cables, try that instead of HDMI and see if it looks better. Maybe the HDMI port on the PS3 is wonky.