Q. Can I order by a tailor a suit during the 3 weeks take then measurements and try on the unfinished or finished suit during the 3 weeks or the 9 days? I will only make sheheyanu when I use it first time for a wedding after T’beav.

A. Shulchan Aruch (O. H. 551: 17) rules not to recite shecheyanu bracha during the three weeks. Sefer Chasidim (840) and Mogen Avrohom explain that it is not proper to say this brocho during times of distress and suffering. Therefore, Poskim restrain the buying of new clothing and items on which one would recite this blessing (Shiurei Knesses Hagedola 12, Bach end of 551, Kitzur Sh. A. 122: 2).
Consequently, since one does not recite this brocho when placing an order for tailoring a suit it is not prohibited.
However, during the nine days it is prohibited to sew or stitch new clothing. (Shulchan Aruch O.H. 551: 7)/
Therefore, Horav Shlomo Miller’s Shlit’a opinion is that one may order a new suit during the three weeks and try it on and fix it, only before the beginning of the nine days, but not afterwards.

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