Customizable 3-prong Telescope Center Dot Placer better for some mirrors.

Decided to add this as a separate file, this is a 3-arm version of my originally 4-armed telescope center dotter. This is because one of my scopes is glued in partially to a holder which has 3 arms, and well, I had to cut off one of my arms to make it work.

arms= does allow for 4 arms (use only 3 or 4, or it will give you not usable results)

Important values:
mirror_diameter (in mm), center_dot_diameter (in mm), center_dot_hole_diameter (in mm)

For reference, standard US, Office Depo