I'm not anything close to an expert...I'm on my first hydro grow, but I'll still throw in my 2 cents lol.

First, if I understand correctly, you've got 9 plants in a 3 gallon bubbler? I've got 6 plants in a 13 gallon DWC system and I've had people tell my that my res is too small for that many plants. Maybe they're too crowded in there? Not sure on this tho. It's just a thought.

Second, if these are in full veg, 300ppm is probably too low. I've got mine running at 600ppm using the Lucas Formula and the plants are loving it. Also, check your res temps. Even if your pH is right, if the res temps are too hot, the plants can't eat. Same with room temps.

Last you've definitely got a nute imbalance somewhere in there. I'm not good enough to tell you exactly what, but if you post in the sick plant forum, there's a guy named Ganja Guerrilla who's really good at figuring out what nutes you're missing, just by looking at your pics.

P.S. The Lucas Formula is a really simply nute program designed specifically for weed. I switched to it in my second week of growth and I'm really glad I did so.