I first plotted the three points and from from their position it was clear which pairs to join to start a rectangle.
At this point you need to check to make sure the angle at B is a right angle. Find the slope of the line segments AB and BC and check that the product of the slopes is -1.
From the diagram you can now see where the fourth point D has to be. If AB and CD are parallel then D must be 3 units to the left of C and 2 units above C. Find the coordinates of D and then check by finding the slopes of CD and DA and showing that the angle at D is a right angle.
I hope this helps
