Can one practice playing an instrument during the three weeks?
In general it is better not to practice playing musical instruments at all during the three weeks, however there are instances It depends on why it is being practiced. If it is because the person wants to earn a livelihood from playing, then it is permitted according to R’ Moshe Feinstein zt”l, however if it for a hobby then it is for enjoyment and not permitted, especially if you are not just a beginner.
Igros Moshe O:CH 3-87, Emes L’yackov ftnt. 508, Kaf Hachayim 551-41, M’bais Levi 13-9-6, See Ashrei Ish O”CH 3 chap. 68-9 in the name of R’ Eliyashiv zt”l that if the person is still a beginner, nd it is still difficult to play, however after the person already knows how to play, then it is an enjoyment and not permitted. Also see Kovetz Halachos (Bein Hamitzarim) 4-10 ftnt.13, who says a similar idea.