Thanks a lot for the reply,

I got it to work by specifying like this.
I was making a fundamental mistake of thinking something else when infact the name of the array is a pointer itself.

void getNormal(vector3D normal, point3D p1, point3D p2, point3D p3)

void main()

vector3D n;
getNormal(n, p0, p1, p2);

Thanks again.