how much should i flush there in three gallon pots
Flushing generally requires 3x the amount of soil you have.
3 gallons soil = 9 gallons water (ph balanced)
Immediately after flush, add 1 more gallon water with your nute mix, since you just flushed all nutrients... Maybe cut a little back on the nutes. And remember the FF's nute table's are NOT for cannabis. You can use the FF's tables as a guideline, but you should always use less.
Oh, and do a RIU search on "molasses" and "hydrogen peroxide". They are both very beneficial to cannabis growing.
Anyone more experienced please correct me if I'm wrong. But, this is what I've learned so far from this site...