Naked Triplet
A Naked Triplet is three cells in one row, column, or box whose combined candidate set contains exactly three digits. Those cells reserve the three digits, allowing the digits to be removed from other cells in the sector.
What is a Naked Triplet?
Take the union of the candidates in three selected cells. It must contain exactly three distinct digits; the individual cells can contain two or three of them and need not have identical notes.
How to spot it
Find three cells in one sector whose candidate union has size three. Then check the other cells in that sector for those digits. First apply any simpler single or pair already present in the same state.
The logic
Three cells require three different values and can draw only from a three-digit union. They occupy all three digits between them, so those digits can be removed from every other cell in the sector.