The main problem here is to put the text into equations. So let's do that first.
x= first pipe water
y= second pipe water
z= third pipe water
In order to avoid any discussion with 0 divisions, we suppose that the amount water we get from the pipes is different from 0.
For the pool I suppose that it is empty when the result of an equation is 0. If it's full, it's 1.
In that case we get:
a is the time in hours we need to fill up the pool with the third pipe.
From now on, it should be easy:
(1) --> y=1-2x
(2) --> z=1-2y
Put into (3):
you will get x=1/4 (you will eventually need to substitute an y with (1).)
this helps us to find y in (2): y=1/2
Put these two results in (3) again and you get a=0. You actually don't need the third pipe anymore