During the 3 weeks or even during the 9 days, is there any prohibition of guys getting together on shabbos and having an oneg, tisch, singing, dancing, etc.? (in honor of shabbos and a future wedding)
During the three weeks this is certainly fine.
In the nine days, there are poskim who write that such “extra” joy should be avoided (see Mishnah Berurah 552:22, citing from Magen Avraham). Citing from Bechor Schor, the Mishnah Berurah writes that those who get together with friends every week should do so even on Shabbos Chazon, because otherwise it would be considered aveilus befarhesia. Again, this refers to somebody who is used to such celebration every Shabbos, and not to somebody who wishes to make a special celebration on Shabbos Chazon.
However, Rav Moshe (Iggros Moshe Orach Chaim 4, no. 112:1) writes that that even if somebody is not used to singing each week, it is permitted to sing zemiros on Shabbos Chazon in honor of Shabbos. Therefore, based on Rav Moshe, it is permitted to do so.