There is only one way to remove a square, aside from rotations and reflections. To see that there is at most one way, do this: Label all the squares of the chessboard with A, B or C in sequence by rows starting from the top:
A B C A B C A B
C A B C A B C A
B C A B C A B C
A B C A B C A B
C A B C A B C A
B C A B C A B C
A B C A B C A B
C A B C A B C A
Every piece must cover one A, one B and one C. There is one extra A square, so an A must be removed. Now label the board again by rows starting from the bottom:
C A B C A B C A
A B C A B C A B
B C A B C A B C
C A B C A B C A
A B C A B C A B
B C A B C A B C
C A B C A B C A
A B C A B C A B
The square removed must still be an A. The only squares that got marked with A both times are these:
. . . . . . . .
. . . . . . . .
. . A . . A . .
. . . . . . . .
. . . . . . . .
. . A . . A . .
. . . . . . . .
. . . . . . . .
