This does take a bit of finangling and is one of the trickier puzzles in the game- at least until you do the math.
Use the three twice to get the 10 up to the 6 meter mark. Now take the 5 and put that in the 10 gauge. Once you do this press down to empty the 10 gauge.
This should leave you with 1 liter left in the 5 gauge- empty that along with two 3's in to make your 7.