Fill the 3L jug, then pour its contents into the 5L jug.
Fill the 3L jug again, then pour as much of it (2L) as you can into the 5L jug.
Now the 5L jug is full, and the 3L jug is carrying 1L of water.
Empty the 5L jug.
Pour the 1L of water from the 3L jug to the 5L jug.
Finally, fill up the 3L jug, then transfer its water into the 5L jug. It now has 4L of water.
