Multiplayer in MW3 is the only place where there are some options for split-screen, but it's not available in every multiplayer mode. Namely, Ground War, Free-for-All, and Invasions modes in multiplayer don't have split-screen available, but the rest of the modes include it.
To access this overwhelming battle, just open up a private match on Modern Warfare 3, navigate to the team and player section to the right of the screen, hit the '+' and add as many bots as you see fit.