Didn't watch that game, but for the record, the C's actually have a worse 3pt% in the regular season than the Wizards.
Sunday, the C's shot 48.7% (12.8% over their normal 35.9%), and the Wizards shot 43.5% (6.3% over their normal 37.2%).
The Celtics defend the 3 a lot better than the Wizards do, so Boston outperforming their season averages are more likely than Washington doing so against Boston.
The Celtics are 2nd in the league allowing opponents to shoot 33.2% against them. Since a lot of statisticians believe that 3 point shooting percentages against are random I'd also like to note that Boston gave up the 7th fewest 3 point a game.
Washington on the other hand tied for 20th 3 point against % in the league giving up 36.4%. The Wizards allow the 17th most 3's against them.
Since our defense has been very good against the 3 and there defense has been very bad I'd argue that those numbers had a ton to do with defense and less to do with luck.
Here is a relevant quote from Celticsblog's Alex Kungu
And for those of you thinking, “well, the Celtics aren’t just going to keep hitting the three like that every night are they?” Probably not, but consider this. The Celtics were 19-39 from three in game 1. Of those shots 39 shots, 33 of them were open or wide open and according to NBA.com, and the Celtics made 17 of those attempts, meaning a majority of those made 3’s were actually just the Celtics hitting open looks
Washington hasn't been good defending the 3 this season and that continued on Sunday.