I do not know. In my circles, the bishop and knight are not always "equal" to exactly 3 pawns when they are being exchanged.
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.
It is worth reading this Atlantic article for the general idea.
Addtionally, the the Wiki page notes many material valuation considerations and alternatives that include fractional pieces.