3 hectoliters = ~ 79.2 gallons, or ~317 quarts.
First wort gravity = (3 hl/100 kg malt)(20 Plato)/(3.2 hl/100 kg malt)
First wort gravity = 18.75 Plato
Long story short - here's an ultra-simple calculation that you can do to figure this out without exploding your brain:
Replace X with your quarts per pound ratio (1.5, for example), and it will spit out the gravity of your first runnings in degrees Plato. Ta da!
In Excel speak, =SUM(0.6)/(B1/48)