I don't see a viable third party.I do see a divorce between the small government end and the socially conservative end coming. But I don't think we end up with three big parties. I think we end up with two big ones and another small party that occasionally gets someone into office like the libertarians and greens do today.
There may be one election cycle where it looks like we may have three, but one of them will quickly become much smaller and less viable and I don't see that being the small government/fiscally conservative part of the split.
Herman Cain is already talking splitting the Republicans and I think they may be to the point they are ready to cut the radical right socially conservative loose and see if they can nab enough of the center from the Dems and get the libertarian types really on board with them.
A lot will depend on the rebranding, how damaged is the Republican brand and is it better to try to shift the GOP or let the wackier end have it and start the new coalition somewhere else.