Hi Cemortan_Tudor, sorry the part I don't understand is if you want to make a mirror of an existing rectangle why not use the Transform > Mirror command (or Transform > Copy) to do it rather than rectangle 3 pts?

However if you do want to use a 3 point rectangle for that you can use Construction lines to do point mirroring. When you place a construction line the mirrored end is available as a snap point. Here's an example:



- Michael