Determining valves for partial draining

I want to drain 1/3 of my 15,600 gallon pool and refill it to bring down the stabilizer level. I plan on using my pool pump in backwash, because I think it can do the job fine, and the hose run to the street for a submersible pump would be well over 100 feet. I've determined which two valves shut off the skimmers, by closing and opening each one successively, but not sure how to verify the bottom drains are open and running. There are three identical valves next to each other, one each for...

Determining valves for partial draining