Hidden triple
Three values confined to three cells lock those cells
How to use it
Three symbols of one unit whose possible homes all lie inside the same three cells. The three symbols are then confined to those cells, and each of the cells loses every candidate outside the trio.
Triples hide better than pairs because the host sets overlap only partially: look for three symbols whose host sets union to exactly three cells, not for identical marks. On sparse units (many empty cells, few candidates) the host-set table makes them visible.
Why it is sound
If three symbols of a unit are possible only at the same three cells, those cells carry the three symbols in some order in every solution. Striking every other candidate at those cells discards no solution.
A worked example
In column 7 the digits 2, 3 and 6 have all their possible homes inside the same three cells — r1c7, r5c7 and r7c7. Between them those three cells take exactly 2, 3 and 6 for the column, so every other digit candidate there is dead weight: 1 and 4 are struck from r5c7 and r7c7, and 1 from r1c7. The pattern hides behind partially overlapping host sets: group each symbol of a unit by where it can still go, and look for three symbols whose hosts union to three cells. This example runs with the easier subset and pair techniques switched off, so the triple gets to do the work.