Question

By how much does the H20 pressure exceed atmospheric pressure at 3 meters below the surface of a pool with fresh H20.

I don't know what equation to use.

You have to figure the weight of a column of water of some height h, and horizontal area A.

Weight= mg= density*volume*g
=density*hA*g

then
Pressure= WEight/A= density*h*g

There is a rule of thumb you ought to memorize: 10 meters of water is about equal to one atmosphere of pressure.