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

Hence all other candidates can be deleted from these three Cells.

In the example above, candidates 3, 4 and 9 are only allowed in cells A6, A7 and A9 of Row "A": three candidates for three cells. No other candidate can be the solution in any of these cells.

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