How to?

Jeff is a poor guy, his boss always like to test on him. If he can not do, then punishment will be applied to him
One day, Boss ask Jeff come to his room and give him 2 empty bottles.
The boss told Jeff the volume of the empty bottle is 5L & 6L, and he would like Jeff to get 3L of water by using this 2 bottles.

Can you help this poor Jeff to solve this problem?