1. The official response from Republicans will come from Rep. Cathy McMorris Rodgers, who will be playing the usual role of up-and-coming Republican looking to not humiliate themselves on national television. It's a tightrope act, and sometimes (we're looking at you, Bobby Jindal) does not live up to the party's expectations.
2. Ah, but ever since Bush-era conservatism collapsed under the weight of, well, the Bush administration, we've been pretending that something called the tea party movement is an entirely separate party that just happens to be made up entirely of Republicans and is spoken for in Congress entirely by Republicans, and so they have decided they need their own "response" to the State of the Union and to the traditional Republican "response." The winner this year is Sen. Mike Lee, primarily known for his role as Sen. Ted Cruz's footstool. His role and agenda will be exactly the same as those of the official Republican response, but Sen. Mike Lee will say it more dully and with less finesse. You know: tea party.
3. The third "response" will be given by Sen. Rand Paul, and will be representing the faction of the Republican Party known as Rand Paul, delivered for the benefit of Rand Paul. Given that Rand Paul really and truly is his own faction of the party (he certainly, say, doesn't give a particular damn about anyone not named Rand or Paul) it's hard to argue with this, and since we all know Rand Paul would be giving a "response" to the State of the Union in front of his bathroom mirror that night anyway, might as well slap a camera in front of it.
So that's three, with more possibly to come. Gotta make sure the conservative point of view is adequately represented, after all, what with the Democratic dominance on all of the Sunday shows.