It depends how high you want the water to fill, but let’s say it’s to 6 inches. That would be 3 x 3 x 0.5 cubic feet or 4.5 cubic feet. A gallon of water is roughly 0.15 cubic feet so I would assume 30 gallons to fill the table, but you want some additional water in your res because you can’t drain it to zero or your pumps will go bad from sucking air.
I would guess probably 40 gallons, but check my math.
I use a 10-gallon reservoir for my 4x4 flood drain table. However, I top feed and let gravity drain back to the reservoir. Less water = less nutrients = less $$$. Every 4 hours the pump turns on for 3-mins.
I grow just 2 plants ( 2 -4"x4" rockwool) on a 2x2 flood table. 6 gallons for me works fine. I would say 8 to 10 gallons in a 12 res would be fine for you.