Hidden Triplet Walkthrough

A Hidden Triplet is three digits confined to the same three cells of a unit. Those cells must contain those digits, so every other candidate can be removed from them.

Sudoku board with pencil-mark candidates before applying the Hidden Triplet technique
Before — candidates pencilled in. There is a Hidden Triplet in Column 3.

There is a Hidden Triplet in Column 3. Applying the Hidden Triplet eliminates 8 candidates (struck out in red below; any solved cells shown in green).

The same Sudoku board after the Hidden Triplet technique eliminates 8 candidates
After applying the Hidden Triplet.