Why not just put the 3 gallon container in the 5 gallon container, fill the 5 gallon one up with water and remove the 3 gallon one whichgives you 2 gallons in the 5 gallon, pour that into the 3 gallon and repeat the process. that will give you 2 gallons in each one so you dont hurt yourself carrying it, because you know if they were going to kill you over 4 gallons of water, they are gonna make you carry it!