First transfer angle between 3 points to angle between to vectors.

Assume you have 3 points: A(xa,ya), B(xb,yb), C(xc,yc)

And if you want to find angle between AB and AC

1. create two vector AB=(xb-xa,yb-ya), AC(xc-xa,yc-ya)

2. inner product between two vectors AB AC =|AB|*|AC|*cos(cta)

You can cllculate cos(cta)= inner product of two vector/ (|AB|*|AC|) to find angle cta

where |AB| is the length of vector AB.

I think you know how to do the sorting.