|||||||| |||||||| |||||||| ---***+* ---...+* ---+O+ ---*+... ---*+***
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
ABCABCAB CABCABCA BCABCABC ABCABCAB CABCABCA BCABCABC ABCABCAB CABCABCA
Every triomino 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
CABCABCA ABCABCAB BCABCABC CABCABCA ABCABCAB BCABCABC CABCABCA ABCABCAB
The square removed must still be an A. The only squares that got
- marked with A both times are these
........ ........ ..A..A.. ........ ........ ..A..A.. ........ ........