DeBerg, a senior from Edina who shot 1-for-6 from beyond the arc in St. Thomas' previous two games, hit the three-pointer to pull the Tommies (26-1) even in the first half, then made five in a row after halftime to put the Auggies (21-7) away.
"Once you hit a couple in a row, it seems you can't miss," DeBerg said.
St. Thomas, the eight-time defending conference regular-season champion, earned its record 12th MIAC playoff championship, including seven of the past eight. The top-ranked team in Division III is hoping for one of two byes when the 62-team NCAA tournament field is announced Monday.
Dan Kornbaum had 20 points and 10 rebounds for Augsburg, which must wait to see whether it receives an at-large bid to the NCAAs.