Playing ME3 co-op on a single PS3
Both console versions of the game lack support for local and system-to-system multiplayer. You'll have to play via the internet. You'll need 2 copies for this to work.
Co-optimus is the best source for this type of info, here's the Playstation 3 page for ME3, and the XBox 360 version.
Additionally, the difficulty doesn't really scale down with fewer players, so you might consider picking up some extra players over the internet if you want to play. Otherwise it is likely to be quite hard.