Three digits confined to the same three cells in a unit; all other candidates in those cells are eliminated.
When three digits can only appear in the same three cells within a unit, all other candidates in those three cells can be eliminated. It's harder to spot than a Hidden Pair since the three cells may have many other candidates obscuring the pattern.
| 8 | 3 | 6 | 1 | 2357 | 4 | 9 | 2 | 5 |
| 2 | 9 | 4 | 7 | 13689 | 5 | 3 | 8 | 6 |
| 5 | 1 | 7 | 9 | 2457 | 6 | 8 | 4 | 3 |
| 4 | 6 | 3 | 8 | 12679 | 2 | 5 | 9 | 7 |
| 9 | 7 | 1 | 2 | 3458 | 3 | 6 | 5 | 4 |
| 6 | 8 | 2 | 4 | 2457 | 7 | 1 | 3 | 9 |
| 3 | 4 | 5 | 6 | 3578 | 1 | 7 | 6 | 2 |
| 7 | 2 | 9 | 3 | 15679 | 8 | 4 | 1 | 8 |
| 1 | 5 | 8 | 5 | 2348 | 9 | 2 | 7 | 1 |
Column 5: Digits 1, 6, and 9 only appear in R2C5, R4C5, and R8C5 → Remove all other candidates from those three cells.
This 9×9 puzzle is solver-verified to require this technique on its solution path.