Test Script

using UnityEngine;

public class Test : MonoBehaviour {

    void Start () {
        Vector3[] coords = {new Vector3(2,1,0), new Vector3(1,1,0), new Vector3(1,2,0), new Vector3(0,0,0)};
        coords[3] = Find4th(coords[0], coords[1], coords[2]);
        foreach (Vector3 v in coords) {
            new GameObject().transform.position = v;
        }
    }

    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);
            int ai = (i != 0) ? i-1 : 2;
            int bi = (i != 2) ? i+1 : 0;
            Debug.Log($"{i}: {ang}, {ai}, {bi}");

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

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

        Debug.Log(cornerA + cornerB - point);

        return cornerA + cornerB - point;
    }
}