Q. Can someone that works creating and assembling stained glass windows and selling the commercially do so during the 3 weeks? What happens if the customers are non-observant Jews?

A. Horav Shlomo Miller’s Shlit’a opinion is that it is permitted since the artist or artisan does not necessarily especially enjoy his art work, sine it is being done as a monetary remunerating work.
The Rov maintains that the work can be sold or created for anyone who pays for it.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a