I was curious, so I looked up the top 25 and bottom 25 teams in defending the three.

Top 25 -

286-125 record 70.4%

Only 1 team has a losing record at 7-12

Mean 3 pt FG% allowed is 29.5%

Bottom 25

157-304 record 28%

Only 5 teams have a winning record, with three at 9-8, 9-8 and 9-7

Mean 3 pt% allowed is 39.3%

So, about a 10% difference between bottom and top.

20 attempts per game x 3 = 60 potential points

60 x 29.5% = 17.7 ppg
60 x 39.3% = 23.58 ppg

That is right at a 6 point difference per game gained by being a top three point defensive team.