Finds the 4th point in a rectangle, given three known points

Find4thPoint.cs

using UnityEngine;

public class Find4thPoint {

	public Vector3 Find4th(Vector3 a, Vector3 b, Vector3 c) {

		Vector3 point, cornerA, cornerB;

		Vector3[] points = new Vector3[] {a, b, c};
		float angleDelta = 180;
		int pointIndex = -1;

		//find point with angle closest to 90 degrees
		for (int i = 0; i < 3; ++i) {
			point = points[i];

			cornerA = points[i != 0 ? i-1 : 2];
			cornerB = points[i != 2 ? i+1 : 0];

			Vector3 lineA = cornerA - point;
			Vector3 lineB = cornerB - point;

			float ang = Mathf.Abs(Vector3.Angle(lineA, lineB)-90);

			if (ang < angleDelta) {
				angleDelta = ang;
				pointIndex = i;
			}
		}

		point = points[pointIndex];
		cornerA = points[pointIndex != 0 ? pointIndex-1 : 2];
		cornerB = points[pointIndex != 2 ? pointIndex+1 : 0];

		return cornerA + cornerB - point;
	}
}