Empty the Bucket Puzzle

You with your few friends are lost in a scorching heat in a desert. You have 3 buckets of water having certain amount of water in all of them (3 ltrs, 2 ltrs, 1 ltr ). Suddenly, you found someone in the desert who is in urgent need of an empty bucket. You decide to give him one of your bucket, but water being precious you have to empty the bucket without any water being wasted away. Also you have the bigger strange constraint while pouring in any bucket

you must double the contents of the bucket which receives the water.

One of your friend finds out the solution for the 3,2,1 problem as something like this
321 — 141 –042

At this the stranger is startled and tells that this is possible for any amount of water in any bucket say a,b,c as far as they are natural numbers. HOW??