generating function

So there are three shareholders and 2n shares to split among them
let $a_n$ be the number of ways the shares can be split such that no single person controls half the total or more

The question asks me to find the generating function A(x) for the sequence $\{a_n\}$

I know $a_n = {{n-1}\choose{2}}$ but I need to find the generating function not the actual closed formula

I've reverse engineered it to be $\frac{x^3}{(1-x)^3}$
But I need to find it coming from the other direction haha

Could anyone explain how to do this?
thanks
JB