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.
C's struggling to defend 3-point line - Boston Celtics Blog - ESPN Boston
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.