If three Cells in a region (Row, Column or Square) contain exactly the same three candidates or only subsets of these three candidates, then one of these candidates is the solution for the first of these Cells, a second one is the solution for the second of these Cells and the last candidate is the solution for the third Cell.

Hence none of these three candidates can be the solution in any other Cell of that region; these three candidates can be deleted from the other Cells of that region.

You can practice this strategy by installing the free SudokuCoach application on your Android™ device.