|||||||| |||||||| |||||||| ---***+* ---...+* ---+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.. ........ ........

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