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Your question is not precise enough. An array of points is only « clockwise » or « anti-clockwise » relative to a reference point. Otherwise, any array of three points can always be either CW or CCW. See the following picture: on the left, the points are ordered clockwise; on the right, the exact same points are ordered anticlockwise.

In your case, I believe using the barycenter of the points as a reference point is reasonable.

A good method for an unknown number of points could be the following one:

• let P[0], P[1], ... P[n-1] be the list of points to sort
• let M be the barycenter of all points
• compute a[0], a[1], ... a[n-1] such that a[i] = atan2(P[i].y - M.y, P[i].x - M.x);
• sort points relative to their a value, using qsort for instance.

However, you can be sure that a good sorting algorithm will perform poorly with three input values compared to an ad-hoc method. Using atan2 is still valid, but just don't use qsort.