I know shytty defense is the main culprit but some of these 3s opponents are hitting are lucky as hell. I mean Jimmy will probably never hit 5/5 again in his life and you got scrubs hitting nothing but net against us. If we can limit those 3s and get a bit of luck, we can win games.
Playing poorly on defense leads to creating the luck that you're talking about.
Defense isn't only about defending the actual 3-point shot attempt...the effects of good defense compound for the duration of the game. Good defense negatively affects the opponents' confidence in all aspects of their game by interrupting their rhythm, increasing their level of mental stress (so they forget their training and fall back on bad habits), etc.
When you're doing those things
and contesting those 3-point shots...chances are they won't be making too many of them. But the contest alone just isn't enough - not with all the focus on improved shooting throughout the league the past couple of years.