There is too many wintraders on blackrock in 3v3 just take a look at the 3v3 ladder. At the time of writing this most of the top ranks are actually obtained by wintraders. These guys play over night when no ones else does and try to get themselves an easy 2,2k rating for the weapons. And what you see there are only the stupid players who didnt straight wintrade to 2,2k and didnt disband their team. There has been a lot of other teams wintrading that you dont even see.
At this point there is probably more wintraders who have the t2 weapons than normal players.
My suggestion would be to make the weapons available through 2v2 too. Restricting such an important item to 3v3 just doesnt work with such a low 3v3 activity and its the root cause for all the wintrading.
Maybe you can add other incentives to play 3v3 instead that are less important than the t2 weapons instead.