3-in plus 1.5-out every minute means a net gain of 1.5 per minute. You need to fill (12 - 3) = 9 gallons. At 1.5 per minute, it takes 9/1.5 = 6 minutes if the drain and the faucet arr both open, but only (9/3)=3 minutes if you close the drain while you fill the tub.
