If you trade baskets and you are letting them hit 3's while you are only getting 2's, simple math tells you that you aren't going to do so well.
Boston's 3-point defense has been thrust into the spotlight in recent games, particularly after the Spurs (12-of-25, 48 percent) and Rockets (10-of-27, 37 percent) bombed away from beyond the arc while sweeping both nights of a Texas back-to-back over the last 48 hours. The Celtics appeared to be settling in with their 3-point defense, allowing opponents to shoot a mere 33 percent from distance during Games 10-20 this season. But the last three games have seen a spike up to 40 percent. Boston must find ways to limit the 3-point damage.
In some respects, you just have to tip your hat to a team that is just making them with high volume. On the other hand, you can do things to rotate and flash to the 3 point line that will limit shooters' open looks. So there's some bad luck and some bad effort going on lately. Hopefully both will turn around soon.