Find the number of positive integral solutions of $x+y+z=n$, $n$ belongs to set of natural numbers, $n\ge3$
How far I've got:
The number of positive integral solutions
= coefficient of $x^n$ in $(x+x^2+x^3+...)^3$
= coefficient of $x^{(n-3)}$ in $(1+x+x^2+...)^3$
After this step what do i do?