Ahem!!... Allow me to give my humble opinion...
It is strange that bishops and knights are given similar mathematical value, i.e. 3 points each in reference to a pawn as 1 point. But I think the value of pieces can change substantially depending on the remaining pieces on the board. Generally speaking, in a closed position when there are still many pawns on the board, the knights are better pieces than the bishops because they can jump over pieces. In this sense, they are more mobile. The bishop can't jump over pieces and can get in the way in a cramp position.
But in an open position where not many pawns are blocking the diagonals, then the bishops are usually stronger since they have longer range, and therefore can attack more squares than the knights.
Occasionally there are positions where there are some pawns on the board and pieces have been exchanged; and one side is left with a bishop and the other has a knight. In such a case, it usually depends on the colour of the squares the bishop is controlling. If for example the bishop is controlling say the dark squares, and his pawns are also on the dark squares, then that bishop is commonly known as the bad bishop because its movement is hampered by its own pawns. Therefore, in such a case, the enemy's knight can be of higher value than the bishop, i.e. knight is better than bishop.
Hope that can help a bit. Now I just need to make some practical use of those ideas in my games... hehehehehe. Nah.. just kidding, those are actually important chess principles which have helped me in many of my games.