Is it mutar to trim the top moustache during the 3 weeks?
It is only permitted to trim a moustache during the Three Weeks (and during the Nine Days) if the moustache disturbs while eating. If the moustache does not disturb, but one wishes to trim it for aesthetic reasons alone, it is not permitted.
Sources: Shulchan Aruch, Orach Chaim 551:13; some maintain that during the week of Tisha B’av it is prohibited to trim one’s moustache even if it disturbs during eating (Kitzur Shulchan Aruch 122:4). However, one may be lenient in this matter (see Maamar Mordechai, 551:13). If the moustache does not disturb eating, it is prohibited to trim it, like all other hair.