Previous versions of rFactor showed triplehead replays. In the final patch (or maybe one patch before that) this feature was removed for whatever reason. AFAIK there is no setting to get triplehead replays.
Make sure you check out rFactor's "MultiView" option, which most people find very helpful in triplehead.
I haven't messed with it. But I did find that Instant Replay does play fullscreen. Well at least if you hit it while drving. I may see if I can find a edit for enabling regular replay in fullscreen. If I do, I will let you know. Seeing the instant replay work, surely there is a way for the recorded replays should.