Step 11 · Hard
What is Hidden Triple? How to Solve Killer Sudoku
When three digits can only go in the same three cells of a group, you can remove all other candidates from those three cells.
What it is
Like a Hidden Pair, but with three digits. In a group, three digits can only go in the same three cells.
Those cells must hold those digits, so remove all their other candidates.
How to spot it
Collect digits with only two or three spots and look for three that share the same three cells.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
In box 7, 1, 3, and 7 can only go in the three cells R7C2, R8C2, and R9C3. Those cells must hold 1, 3, and 7, so remove their other candidates (2 and 8).