Creative director of Insomniac games Marcus Smith confirmed on the PS blog.
This is what he had to say
“In terms of co-op, we are focusing on a two-player co-op through the single-player campaign, which can be played online with a friend, or split-screen on your couch.”
“New to Resistance 3 are upgradeable weapons, which will give your weapons new abilities and power as you continue to use them.
“In terms of multiplayer,” he added, “we are planning a much more focused, progression-based multiplayer experience.”
Looks like the MP will be a blast.