What are the reasons why a person may be able shave and/or haircut (two biiiits!- sorry, just had to) during the three weeks? Please use sources.
Source: Kitzur Shulchan Aruch 122:
During the 3 week you can shave/haircut if:
- You're a Mohel, Sandek or father a boy having a Bris
Until Rosh Chodesh, for somebody whose Shloshim ended on or after 18 Tamuz (KSA 122:14)
You can trim your mustache if it interferese with your eating (KSA 122:4)
The Mishna Berura (OC 551 sk 32) rules that one may shave or take a haircut to look nice for Shabbat if one generally does that action at least once a week.
Two more answers:
If your Safardi, until the week of Tisha b'Av
Clean-shavers who ascribe to R' YD Soloveichik's comparison to the format of yud bais chodesh may shave until Rosh Chodesh.