"Good things kill in threes." is a red text modifier on weapons in Borderlands 2
Found on the following red text weapon: Barbed Triquetra, (or any other Triquetra variants) which you get as a reward from the mission "Clan Wars: End of the Rainbow".
Grants Weapon Effects
Fires bullets in a Triquetra pattern
Consumes 3 ammo per shot but does more damage