Yes, what you wrote is Trichlor. Trichlor is essentially a combination of Cyanuric Acid (CYA) with chlorine. In water, these separate into their two pieces and recombine in exactly the same way as if you had added CYA and chlorine separately to the water. Trichlor is very acidic so it should NOT be used in the skimmer and especially not if the pump ever stops running. I stupidly did that once and it rusted the thermometer in the skimmer and possibly caused damage to my pool heater (and even the pump) when the blast of acid went through the pipes when the pump turned on (I hope it didn't and haven't had problems, but I took a huge risk -- this was all before I learned better from these pool forums and my own investigation into pool water chemistry).
Generally, if Trichlor is used, it is put into a floating dispenser which has it slowly dissolve. It can also used in an inline feeder.
The main downside to Trichlor, other than its acidity, is that it adds CYA to the pool and CYA is not something that breaks down (normally) in pool water. So it continues to build up UNLESS you have significant splash-out, backwashing (if you have a sand filter or regularly clean your DE filter) or have lots of rain that overflows and dilutes your water. Otherwise, you would have to regularly drain and refill part of your pool water to keep the CYA level relatively constant.
I would only use Trichlor if your CYA is a little low and you want to increase it. Right now in my own pool I'm using Trichlor just for that purpose, but when I get to my desired level (20 ppm CYA for my pool since I have a pool cover that keeps out the sun most of the time), I will switch to chlorinating liquid (regular bleach would be OK, too).
