Rubio just finished his victory speech, and his message coming out of South Carolina was clear: There are now officially only three candidates that matter, and that he is best positioned to win. He hit this note at least three times during a speech that was typically long on lofty rhetoric and praise for the recently-departed Jeb Bush.
It's hard to argue with at least the first half of Rubio's point. Carson finished dead last in a state that he was predicting last week he was going to do "very well" in. He looks like he will finish significantly behind John Kasich, who basically conceded the state. Speaking of conceding states, Kasich's strategy of conceding every state until Michigan is insane and absolutely will not work. Sooner or later Kasich will get the message and drop out himself.
The second part gets a little trickier. As of this moment, it looks like Rubio is going to finish second in South Carolina, which is a state that ought to have been more favorable to Cruz. Probably, you have to assume that the lion's share of Jeb's support (what remains of it) will go to Rubio, which will also help.
But the remaining X factor in this race is the looming specter of Trump. You have to figure that if Rubio follows up this second place finish with a second place finish in Nevada (which looks likely), then Trump is likely to unload on Rubio in next Thursday's debate in Houston. The last time he faced such pressure from a brash personality, he wilted under the pressure. How will he respond?
If he's able to stand up under the pressure, then I agree that he likely emerges the victor. If he wilts again, then it becomes a two man race between Trump and Cruz. And no one can know who will win that toss up.