Find the solution to the Laplace equation in \(R^{3}\) :

\[\frac{\partial^{2} u}{\partial x^{2}}+\frac{\partial^{2} u}{\partial y^{2}}+\frac{\partial^{2} u}{\partial z^{2}}=0\]

assuming that

\[u(x, y, z)=f(x) g(y) h(z)\]