I would fill the the 3 gallon jug, then fill the 5 gallon jug half full. I would proceed to drink the contents of the five gallon jug and wait 15 minutes. When I could no longer hold the 2 and 1/2 gallons in my bladder I would release it but stop it before exit. Knowing my anatomy that would be exactly one gallon. Pour the three in the five and add the one, problem solved.