Well... if you really want it in 3, I can try, but I do see a good mate in 2. Adding in the extra move might mess things up a bit, though.
First off, the Black bishop is completely worthless, as the only white piece on the board on a black square is the knight, and he will be moving... even IF the bishop decides to find a square from which it can interfere, the option is White's as to wether or not to allow the interference (and, obviously, White will not).
The Black King is frozen, and simply cannot move, even given the absence of the knight... for this reason, it is simply considered a static target. Given that every square around the king is death, any attack on the King results in mate.
The Knight's support of the rooks is completely valueless as well, since they effectively support each other quite nicely.
The White King's position is also going to remain stable, as he provides a very nice anvil on which to crush the opponant king.
This leaves the knight as the only piece on the board that needs to move. Assumably, the bishop will be the king's source of (feeble) resistance.
xx Kb3 or Kf3.... from either of these squares, there are two possible mating moves (c1 or c5) or (e1 or e5)...the choices here are really based on what the bishop does... for example, if the bishop holds the same diagonal, then obviously, e5 is out... but even in the few cases like this, you simply take the alternative... here, e1. In this vein, probably the best square for Black's bishop is d4... right between the twin hammers of White's rooks... yet, again, both c1 and e1 are open. Similarly, if the bishop chooses to roost at b2, then both c5 and e1 remain possible.
After the knight has picked its side (kingside or queenside), the best that the bishop can hope to accomplish is picking the square that the knight will crush his king from.
Mate in 2.
This position it will not happen in real game
maybe black resign before this positon happen...
i put this for brain twisting and for fun only GOOD LUCK!!!
POST YOUR SOLUTION AND I'LL PUT THE SOLUTION LATER
white to move
mate in 3 moves