Dagger Shot

Dagger shot is a made shot, sometimes a three-pointer, in a pivotal part of the game; e.g. a shot that silences a rowdy crowd, puts the team ahead in the closing moments of a game, discourages the opposing team, or kills their confidence.

Usage of the term dagger for any long-distance shot is a bit hyperbolic, given that the classic dagger is a climactic event, not something that happens multiple times in one game. But the term has come to be used to refer to a long-range shot that discourages the opposing team or kills their confidence.

The Golden State Warriors’ radio play-by-play man, Tim Roy, by the way, used the term dagger much as the commentators quoted by the OP do: for a lethal, momentum-killing shot. Presumably the fact that a dagger can be thrust by hand or thrown some distance with lethal effect makes it a suitable metaphor for a killing three-point shot.