Tier 3 · first asked for by Tough
Hidden Triple
Three digits confined to the same three cells clear everything else out of them.
Three digits in a unit whose possible homes, taken together, come to exactly three cells. Those three cells are spoken for, so every other candidate in them can go.
As with a hidden pair, the cells involved may look busy — that is the point. The deduction is about where the digits can go, not about what the cells contain, and it usually strips several candidates at once.
A worked example
A real position, with the pencil marks a solver would have at that point.
the pattern · what it rules out · 5 a candidate that goes
What it proves. In row 4, 1/3/9 can only live in R4C1, R4C3, R4C8, so those cells hold nothing else.
Builds on Hidden Pair and Naked Triple.