I think it mostly depends on the position. The value of pieces vary with different pawn placements and scenarios.
Two bishops vs. two knights

It depends. If the position is more open, the bishops are better.
If the position is more closed,the knights are better.

Thanks. This is also a position where the pawns are on "one side of the board." What D'Vivre said (also posited by Silman) is that Knights are clearly superior in this position.
Can you draw up a diagramme please of locked pawns, 3 on each side, with pawns on both sides of the board now?

Thanks. This is also a position where the pawns are on "one side of the board." What D'Vivre said (also posited by Silman) is that Knights are clearly superior in this position.
Can you draw up a diagramme please of locked pawns, 3 on each side, with pawns on both sides of the board now?

Whoa...the thread is about 3 pawns for black, 3 for white. Not 3 each on each side. That's 6 pawns each.

@ Huckebein: Well, it's certainly pretty strong, as seen in that Botvinnik-Bronstein endgame from the 1951 match (where Black was even a pawn up, yet lost).
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A single knight can certainly be a lot better than a single bishop, but with 2 Bs vs 2 Ns the hoppers will definitely have their work cut out for them.
Do you think two bishops against two knights is a decisive advantage in an endgame with three pawns black and three pawns white?