Hidden Triple
Three digits whose only possible cells inside a unit are the same three cells. Those three cells must hold those three digits — clear everything else from them.
Intermediate sudoku technique
What it is
A hidden triple is the three-digit analog of hidden pair. Inside a single unit (row, column, or box), three digits have candidates restricted to only three cells — the same three. Each of those digits must appear in one of those three cells, so collectively the three cells hold those three digits — and only those three. The triple is 'hidden' because each of the three cells may still have additional candidates listed (digits which CAN go in that cell from a row/column/box perspective but which won't because three other digits are locking the cell down). The technique's value is that those extra candidates can be eliminated. Unlike a naked triple — where the three cells visibly contain only three digits between them — a hidden triple needs careful candidate tracking to spot.
When to use it
On hard puzzles after naked pairs, pointing pairs, and naked triples are exhausted. Scan each unit by listing which digits have candidates restricted to exactly three cells in that unit, then look for three digits that share the SAME three cells.
Worked example
In row 4, three digits — 3, 6, 9 — each have candidates restricted to only cells R4C2, R4C5, and R4C8 (each of those three cells may also list other candidates like 2, 5, or 8). Since the three digits must each occupy one of those three cells, collectively R4C2, R4C5, R4C8 will hold {3, 6, 9}. Any other candidates in those three cells (2, 5, 8 from the earlier example) can be eliminated.
Try it
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