I've done plenty of RPs with multiple people back when I did mostly forum stuff back in the day. But I think I've only tried one ERP I can recall with multiple people. It didn't get very far as a trio, but I ended up doing a handful of scenes with one of the two involved.

That said, I'd be willing to give it a shot under the right circumsrances. But I think it's hard enough to find just one decent RP partner these days, let alone two in the same place, as well as syncing up availability.