What could be happen to plant C?

Can it also be an issue with them all fighting for root space?

With high roller and you on this. pH fo sho. As for reading your pH, are you able to calibrate this specific meter? If so, a 7.0pH calibration fluid can be used to put that bugger back in check. Two brands of this fluid are blue lab and Growtek (blue lab is expensive though. Go with a cheaper version). If you can't calibrate this meter and/or don't trust it, I highly suggest a manual pH test kit. General hydroponics is a decently priced option and lasts for a long time. I actually do a test with my meter (calibrated) AND a manual test to prove the results I'm reading. If something's off with the meter, the manual test will tell you so and I'd personally take the manual reading over a fallible electronic anyday👍👍 Good luck and don't give up! I trust there is time to bring them all back to health (even the little guy). Cheers👍😉