If you want split screen co-op, get rainbow six black arrow when it comes out. If you only have rainbow six 3, your options are limited to online or system link. In other words, SOL. I recommend Ghost recon or the island thunder "expansion"; it supports split screen co-op and adversarial, with a nice selection of game modes.
I wish more game companies offered split screen co-op, as I've noticed a disturbing trend of dropping split screen and AI bots in favor of online play. Why not have all 3? I hope to god conkers live and reloaded has 4 player split screen play.