A hidden triple is three digits that, within one house, fit only into the same three cells. Those cells must hold exactly those three digits, so any other candidate in them can be struck. It is the hidden pair grown by one — and considerably better camouflaged.
To spot one, sweep the house digit by digit and write down each digit’s possible cells. You are looking for three digits whose homes all fall inside the same set of three cells. As with the naked triple, no digit needs to fit all three cells — the three lists just have to stay inside the same trio.
It works because the house must place all three digits, and only those three cells can take any of them. Three digits, three cells: there is no room in the trio for anything else.
On the board
Sweep row 7 on this board. The digit 3 fits only at columns 5 and 6; the 4 only at columns 6 and 9; the 6 only at columns 5 and 9. Three digits, and every home falls inside the same trio — R7C5, R7C6, and R7C9. Those cells must hold the 3, 4, and 6 among them, so their other marks are struck: R7C5 loses its 7 and R7C6 loses its 1. R7C9, already showing just {4,6}, was the tell all along.
This is the exact board the app's interactive Hidden Triple demo uses — step 17 of its 65-step walkthrough, drawn here with its live pencil marks.
