I give you no better than 50/50 odds of escaping alive, and that depends on you being an expert-level swimmer.
Class III whitewater is nothing to mess with if you have no clue what you're doing. (AKA "The only thing I've used a kayak before is saltwater fishing.") The underlying rocks will create currents that will confound you if you haven't learned how to read them -- within a few minutes, you'll have capsized, and at that point, King Neptune will play with you like a cat toy.
If all you've ever done is gone fishing from your kayak, you most likely don't have a bombproof eskimo roll. A bombproof eskimo roll takes a lot of practice -- to the point where you just don't think about it, like you don't think about turning a doorknob. This means one of two things once you capsize (which will happen within the first few minutes of your trip).
You'll whack your head on an underwater rock and drown.
You'll end up swimming, and that puts you at risk of being smashed against any rocks, logs, or other debris that's in your path. Plus the aforementioned risk of siphons.
In either scenario, you'll put the trip organizers and other rescuers at risk of their own lives when they have to go fetch your soggy state of affairs. Speaking as an EMT, this is no bueno. Nothing is more important to us than being able to come home to our families at the end of our shifts. If we decide the risk is too great, your body becomes unrecoverable.
Consider that if you stand up in a fast moving river, and your foot gets wedged under a rock, the force of the water will push your body down before you know what's happening. At that point, you're dead: no-one is strong enough to lift your head above the water. The hydraulic forces are just too great. If you broadside a rock in a kayak, the water will push the two ends around the rock, horseshoe-style, and you're in the weak spot.
Here's a YouTube video of what can happen in fast moving water that's not even knee deep. Note that the guy got stripped naked by the water, and then three guys get swept away. There's nothing they can do to regain their footing. Note what that water was doing to a 2,000 pound car.
If the river features a rafting outfitter, and they have amazing guides, this could be a great way to have some fun. I did a trip in Costa Rica, we did some Class III and some small bits of Class IV. For reference, the Class III was incredibly fun. (And I say this from the perspective of someone who isn't at all afraid to mix it up.) The Class IV was scary enough that it started crossing the line. There were a couple places where the boat nearly capsized. There were a couple places where people fell out of the boat -- which is really hairy.
If you want to do whitewater, making Class III your very first introduction is the kind of thing you do if you're ready to kiss your mom goodbye for the very last time ever. Take a few classes, get some easier rivers under your belt, work your way up to the big stuff. This isn't the kind of thing you do spur of the moment.