[obs20]

Now that I had a pint of beer I can think straight.

Spoiler:

Fill up the 7 pint jug. Pour it off and fill the 3 pint jug. Empty the 3 pint jug into a convenient drain.
Repeat process.
Now we have 1 pint in the 7 pint jug. pout that into the 3 pt jug to hold it. Fill the 7 pint jug from the large jug. Empty the large jug into a drain. Now pour what's in the 7 and 3 pint jugs into the large jug. There should be 8 pints there which is exactly 1 gallon.