If you flush just use plan PH water at 6.5. Purified water is just water that has been boiled, you can use that. I would drill a lot of holes in your pots, on the side, bottoms, I think you are have a root problem/rot by taking so long to dry out especially in a 3gl pot and a fan blowing right on it, I'd look at this ASAP before a flush.
3gl will get it done but 5gl get it done better with a longer vegging plant(7weeks+) 85f/ 60% humidity is OK for now but after the flip target 74f/30% humidity. I would still move the fan if at all possible.
Hope this helps and will be here for updates.
Good luck