For watering or feeding, one gallon should be fine for 3 plants in 3 gallon pots. (just over a quart each) But there may be times you'll need 2 quarts each. Like the middle of summer, perhaps. For me it's easier to adjust (skip or add) watering days, then to adjust 1 or 2 gallon mixes. I have to adjust my ph every time, and I only mix what I'll use at that time.

But a lot will depend on how the medium retains moisture, (humidity, heat, circulation and ventilation) and how far you let them dry between waterings. Pay attention, (mark a calendar) and you'll figure-out a pattern. Get's tricky with the change of seasons, so keep that in mind too.