May one get a haircut during 3 weeks if there is an issue of chatzitza for putting on tefillin? For example a young man with long hair like a goy is chozer beteshuva during 3 weeks. May he be told to get a haircut during the three weeks so that he can put on tefillin properly and not go around looking like a goy?
If it is really a tzorech mitzva, and his hair is so long that it is a chatzitza, then he may cut it in order to do the mitzvah. See sources.
There are two sources that I know of, on this topic. R’ S. Z. Auerbach in Maadanei Shlomo pg. 169, says that if that is the case he may cut his hair, but only in the place that the tefillin and the straps have to go, even if he will looks silly that way!! Also see Ulasher Amar 18 that it is permitted, (without restriction). Also see Kol Hamisablim Aleha pg. 21