To be quite honest, I wouldn't even waste my time with 3%, it's garbage IMHO you would need a vast amount of it to get the desired effect. Get the 35% or 50% if you can get it. Trust me and ask any old timer in here, get the good stuff.
How are the roots looking? Are they brown or slimey? Root damage is permanent; new root hairs can form, but damaged roots will not regenerate. Lightly infected roots may turn white again if treated promptly.
If it is Pythium, I would maybe try a Hydrogen Peroxide root dunk
-Remove each plant from system, snip off potions of the roots system badly diseased.
-Dip/swish each plant and container into a strong H2O2 solution, until diseased roots have been removed. Repeat as required. The H2O2 should burn off the pythium-infected outer root layers. Sterilize all equipment too and clean the shit out of everything. Add anti-pythium additives, Vitamin B1, and fresh nutrients to a sterilized reservoir at a lower strength.
FloraShield ,Root Shield or Hydro Shield (or any other product along those lines) may help as well.
Good Luck!