Tier 3 · first asked for by Tough
Naked Triple
Three cells in a unit sharing three candidates between them lock those digits away.
The same argument as a naked pair, one size up. Three cells whose candidates, taken together, amount to exactly three digits must use up all three — so those digits leave every other cell in the unit.
The cells need not each hold all three. {2,5}, {5,8} and {2,8} is a perfectly good triple: no cell has more than two candidates, but between them they need 2, 5 and 8 and nothing else. That is what makes triples harder to spot than pairs — you are looking at the union, not at any individual cell.
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. R9C1, R9C6, R9C9 in row 9 hold only 2/4/7 between them, so those digits leave the rest of the row.
Builds on Naked Pair.