The problem is this
we have a box that has in it 3 vessels which have the property that they empty their content once they are full. Their content is being emptied to the vessel to their right. Once the most right vessel is full then it just empties its content out of the box.
Here is a picture to illustrate this a bit better.
The vessels at the beggining are empty.
The only lead i have about it is that out of the box i get either 12 glasses of water or 15.
How many glasses of water is the volume of each vessel??
I can do it with just two vessels, $14$ and $3$. When the $14$ spills, the $3$ will dump four times, releasing $12$ and holding $2$. The next time the $14$ spills the $3$ will dump five times, releasing $15$ and holding $1$. The third time the $14$ spills the $3$ will again dump five times, ending up empty, and we go around again.
Any first vessel that is between $12$ and $15$ will work. The fraction of spills that is $15$ will rise from $0$ to $1$ linearly with the capacity of the first vessel. If I have to use a third I can just put anything less than $15$ in front.