Step 8 · Hard
What is Hidden Triple? How to Solve Sudoku
In short
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
Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.
In box 3, 5, 7, and 9 can only go in the three cells R2C7, R2C9, and R3C9. Those cells must hold 5, 7, and 9, so remove their other candidates (2 and 4).