Let $f(x,y,z) = xyz$ be a function of three variables where $x>0,z>0$ and $y\in[0,1]$. For a function of two variables i.e., $f(x,y) = xy$, the approximation is given as:
$f(x,y) = (x+y)^2 -(x^2 + y^2)$
The above function is a difference of convex and concave function which will be eventually solved using CCP. How can the above analogy be extended to a function of three variables?