1) Ashkenazim maintain that one may not take a haircut from the seventeenth of Tamuz until chatzos on the tenth day of Av. (Rama 551:4) Sefardim prohibit haircuts only during the week of Tisha b’Av,. (Moed L’Kol Chai 6:25, Nahar Mitraim Tisha B’Av 6 and Yechava Daas 3:39)

2) Harav Shlomo Zalman Auerbach zt”l writes that according to the letter of the law an Ashkenazi may give a haircut to a Sephardi during the time period where Sepahrdim are allowed to shave (from the 17th of Tamuz until the week of Tisha B’Av). However, it is preferable for the Ashkenazi to avoid doing so when possible. (Halichos Shlomo Moadim vol. 2 Dvar Halacha page 414)

3) Combing hair is permissible. (M.B. 20 and Shaar Hatzion 28)

4) In an instance where not getting a haircut is likely to cost somebody money, Rav Moshe Feinstein (Igros Moshe Orach Chaim 4:102) allows one to shave during the three weeks because the prohibition of haircuts during the three week  is only a minhag. (See also Halichos Shlomo Moadim vol. 2 page 414) However, during the week of Tisha b’Av, itself, when it is halachically prohibited to cut hair, one would not be permitted to do so even in the event of significant monetary loss.  In an instance where one is unlikely to lose any money by not shaving, but one is uncomfortable going into his place of work unshaven for fear that his co-workers may mock him, Rav Moshe Feinstein (Igros Moshe Choshen Mishpat 93) rules that one may not even shave during the three weeks.

5) One may not shave or get a haircut during this period even if one is doing so l’kavod Shabbos. (M.B. 551:32)

6) Women may get their sheitels cut or styled during the three weeks, as this is not considered to be their hair, but their clothing. (See Netai Gavriel Bein Hamitzarim page 139)

7) The poskim discuss whether one may trim one’s eyebrows during the three weeks. Rav Moshe Feinstein was of the opinion that one cannot trim eyebrows during shloshim (thirty days of mourning), and therefore may not do so during the three weeks either. (See Netai Gavriel Bein Hamitzarim page 130) Rav Shlomo Zalman Auerbach however, maintains that  it is permissible to trim eyebrows (and eyelashes) during the three weeks. He felt that such hair removal is not considered to fall into the halachic category of “haircutting”. (Halichos Shlomo Halichos Shlomo vol. 2 page 414)

8) Rav Shlomo Zalman Auerbach allowed a woman who is of marriageable age to take a haircut during the three weeks (and even during the nine days) for the sake of Shidduchim (Shalmei Mo’ed Chapter 89)

9) Many have a custom not to give a boy his first haircut until he reaches the age of three. The first haircut is called the Upsherin. This custom has sources in the works of the Ari z”l. (See Shaarei Teshuva 531:2 and Shulchan Menachem vol. 5 page 24)

10) If a child’s third birthday falls out during the Three Weeks, when taking haircuts is forbidden, the poskim discuss the proper procedure for the Upsherin. A- The Lubavitcher Rebbe zt”l was of the opinion that one should postpone the haircut until after the Three Weeks. He was not in favor of having the Upsherin earlier before the child reaches the age of three (Shulchan Menachem vol. 3 page 49, vol. 5 page 31) [Many have the custom that a child begins wearing Tzitzis from the age of three. In this instance the child should begin wearing the Tzitzis from his birthday, before the Upsherin. (Likutei Sichos 13 page 297] B- Harav Shmuel Wosner zt”l writes that “technically one can have the Upsherin even during the week of Tisha B’Av. However, the custom is to have the Upsherin earlier before the Three Weeks (even though it is before the child’s third birthday). (Kovetz M’Beis Levi Bein Hamitzarim page 6)