Knight or bishop Vs Three Pawns

There isn't any general rule. It depends entirely on the position.

at the beginning of the game the piece is generally worth more than the pawns. unless the 3 pawns are holding the position together and cannot be broken.

doubled, isolated, backwards, and weak pawns are still counted as material but are of less value than healthy pawns due to thier limitations in movement.

in the end game its a different story, the pawns are generally more valuable than the piece unless they are weak and cannot be protected.

with extremely advanced pawns in the endgame, they are so powerful that not even a rook or even possibly a rook and a minor can stop one of the 3 pawns from queening.