Translating the given problem into equation form:
Let A = volume of prism A
B = volume of prism B
C = volume of prism C
Eqn 1: A + B+ C = 518
Eqn 2: A = (1/3)B
Eqn 3: B = C
There are 3 equations and 3 unknowns, the system of linear can be solved. Using a calculator (preferably Casio 991-ES), yields:
A = 74 cu.ft
B = 222 cu.ft
C = 222 cu.ft