How many pounds in 3 liters?

It completely depends on what those liters are full of. The weight will be different for different substances.

-- If they're empty, they weigh nothing. Zero, zip.

-- If they're full of air, they weigh something, but not a lot.

-- If they're full of water, they weigh roughly 249 pounds.

-- If they're full of stones, or gold, or lead, they can easily weigh a ton or more.