Step 10 · Hard
What is Naked Triple? How to Solve Killer Sudoku
When three cells in a group have only three different candidates between them, you can remove those three digits from the other cells of that group.
What it is
Like a Naked Pair, but with three cells. Together, the three cells' candidates make exactly three digits (say 1, 4, 7).
Those three digits are used up by the three cells, so remove 1, 4, and 7 from the rest of the group.
Each cell can hold just two of them, like 1/4, 4/7, and 1/7.
How to spot it
When a group has three cells with two or three candidates, check whether together they use only three digits.
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, R7C1, R8C1, and R9C1 can only hold 2, 6, and 8. That's 3 cells for 3 digits, so 2, 6, and 8 are used up here (even if we don't know which goes where yet). So the other cells of box 7 (R7C2, R7C3, R8C2, R8C3, and R9C3) can't be 2, 6, and 8.