3 way mitres are only perfect provided the pieces are fully at 90 degrees to each other. If the surface it is applied to it ain't square the pieces wont fit. So here come the instructions again. Cut 3 pieces to a standard mitre with the shortest edge on the inside. Draw this cut point across the trim to the outer edge taking care to draw exactly at 90 degrees to the cut edge. Now cut a compound mitre across each tip from the edge of the pencil mark. Remember to match your cut to the pencil mark (I prefer to cut to the edge of the pencil mark not in the middle). Everything must be done exactly. 2 or 3 attempts and you should get the hang of it. I personally use a pencil file to gently take the sharpness of the joint point as well.