Three's a crowd. A trio of candidates for the Republican presidential nomination took their turns at CNN's town hall in Greenville, South Carolina, but the truth is that tonight, at least, only one of them deserved to be there. That would be Sen. Ted Cruz, R-Texas, who won the Iowa caucuses and is polling second in South Carolina's primary that's taking place Saturday.Were this not such an extraordinarily volatile year in Republican presidential politics -- thanks to Donald Trump -- it is likely that both Sen. Marco Rubio and Dr. Ben Carson would have ended their campaigns by now. Instead...