I dunno. In my circles, the bishop and knight are not always "equal" to exactly 3 pawns when they are being exchanged, e.g.
So for us, we might say a player is "a half-piece down" after a bishop for knight exchange (if material was otherwise equal), or vice versa (depending on when the exchange happens), or even "three and a half down" if the player blunders their higher valued minor piece.
In this scheme, at the start of a game, a knight may be considered worth more than a bishop (3.5 while the bishop is 3); while later on, the values switch and the bishop becomes worth more (3.5 while knight becomes 3). Thus, in total evaluation, you get a "half piece" difference.
Of course, this is generally only considered when a bishop for knight or knight for bishop is exchanged, or a minor piece is sacrificed or blundered; or perhaps when evaluating the strength of positions at the end game (as some folks may consider the bishops more useful in the end - but I'd say, the positioning determines which would be better in any situation).
Worth scanning for the general idea: https://www.theatlantic.com/entertainment/archive/2014/11/knight-vs-bishop/383202/
And here the wiki page notes many material valuation considerations and alternatives that include fractional pieces: https://en.wikipedia.org/wiki/Chess_piece_relative_value