Ok If they are symmetrical I would measure how high the 3 gallon goes on the 5 gallon and make a mark leaving 3 gallons below the mark and 2 gallons above the mark. Then I would place then 3 gallon on the line a mark where the top of the 5 gallon is on the 3 gallon then i would ill up the 3 gallon up to the line twice and dump it in the 5 gallon. And presto! 4 gallons*Note this only works if they are both as deep and as wide.* what do i win?