Trades is one of the most important features in chess. Botvinnik said something like "chess is a game of inner-related exchanges"
I have moved away from teaching value of pieces as static. I introduce their static value but stress that value is based on action. so the value of a bishop is something for each player to evaluate based on the position. Rooks in the corner are 1? 2? 3? your choice but its in relation to the activity of everything else on the board. A captured piece is worth ,...0!
The reason i think this is more beneficial than using a static value is pieces are dynamic units. A Bishop is worth more in an open position than closed one. Rooks are more valuable on open files.
so to answer your question
1. depends on the position
2. depends on the position
3. depends on the position
In terms of these unequal (pointwise) trades, would you play them in a game?
1. Bishop-Knight for a rook
2. Rook-Bishop for queen
3. 3 pawns for bishop OR knight
Please feel free to add more *REASONABLE* and i cant stress that enough trades that might occur in a game. Thanks and continue to post in my forums!