Hidden Triplet
A Hidden Triplet appears when three digits can only go in the same three cells inside one house — a row, column, or 3×3 box. Those three cells have to hold all three digits between them, so any other pencil-mark candidate sitting alongside them in those cells is dead weight and can be erased.
What is a Hidden Triplet?
A Hidden Triplet is the three-digit version of a Hidden Pair, and like its smaller cousin it's buried under extra candidates. The three cells that share the locked digits also carry other pencil marks, so you have to hunt for the digits that only appear in those three cells, not the cells that only have three candidates. Once found, the payoff mirrors a Hidden Pair: you strip the surplus notes from inside the triplet cells, not from the rest of the house. It's the mirror image of a Naked Triplet, which locks three cells to three digits and cleans the house instead.
How to spot it
Pick a house and, for each digit, count how many cells can still hold it. When three different digits each have exactly three candidate cells left, and those three cells are the same set, you've found a Hidden Triplet. It's harder to spot than a Hidden Pair because you're juggling three digits and three cells at once, and the extra candidates in those cells camouflage the pattern. Scanning by digit rather than by cell is the reliable way in. Hidden Triplets tend to appear in harder puzzles once the easier subsets have been exhausted.
The logic
Every house has to place each digit 1–9 exactly once. If three digits can only go in the same three cells in a house, those three cells must contain all three digits between them — one digit per cell — because none of the three appears anywhere else in the house. That fully commits the three cells: they'll hold exactly those three digits, in some order. Any other candidate note inside them is impossible, so it can be crossed off. The same reasoning with two digits gives a Hidden Pair, and with four, a Hidden Quad.