Reference it it terms of a right triangle. There are several ways to figure it.
Take you 2 and 1 and they will be the base and the height of the triangle. So if you go 3 foot deep thats heighth and the base will be 6. You then have base and height. That will be area multiply times length of the trench and that will be one sides volume. Double for the other side. Then take then take the actual trench bottoms width times the 3 foot height( or depth) and multiply by length of trench.
I am not great with exceel but I will try to right a program. Should not be too hard.
(Trench depth x slope)x trench depth x length will equal slope volume in cubic ft on one side.