An equilateral triangle with integer side length $n\ge 3$ is divided into $n^2$ equilateral triangles with side length 1 as shown in the diagram below.
The vertices of these triangles constitute a triangular lattice with $\frac{(n+1)(n+2)}{2}$ lattice points.

Let $H(n)$ be the number of all regular hexagons that can be found by connecting 6 of these points.

For example, $H(3)=1$, $H(6)=12$ and $H(20)=966$.

Find ${\displaystyle \sum _{n=3}^{12345}H(n)}$.