Fill the 3L containerPour all the milk from 3L container to the 8L oneFill the 3L container againPour all the milk from 3L container to the 8L one. Now there will be 6L of milk in the big containerFill the 3L container againStart pouring the milk to 8L container. Stop when the big container is full.In the small container there is now 1L milk. You can discard all the milk from the big one.