Brian Monetti, I <3 math!
The volume of cement will be Length X Width X Height. You will need all three measurements in the same units, so if you are using feet you will need to convert the inches to feet. In this case, 3 inches will become 1/4 of a foot, because 1 foot equals 12 inches, so 3 inches to feet is 3/12, or 1/4. Hope this helps!