The value of the pieces

1 knight = 1 bishop = 3 pawns 
1 rook = 4.5 pawns 
1 queen = 3 minor pieces = 2 rooks = 9 pawns

However, the value of the pieces is not an absolute and constant number.

A pawn on the seventh rank can be very strong and perhaps promote to a queen. A knight in the center or a rook on an open file are far more effective than their passively positioned counterparts.

G.Lisitsin described five factors which influence the relative strength of the pieces:
1) The central position of a piece
2) A safe, well protected position
3) The activity of the piece
4) Coordination with other pieces
5) The mobility of the piece

This is true, so much so that in many cases, sacrificing pieces of higher value for pieces of lower value end up leading us to victory

Chess engines sometimes use the PeSTO's evaluation function --> https://www.chessprogramming.org/PeSTO's_Evaluation_Function

and a rook for a knight and a bishop are also a good trade