When three cells in a region together hold only three digits, those digits belong to those cells.
Where to lookA pointer first: in the bottom-left box, 3 cells can only hold the same 3 digits.Naked tripleIn the bottom-left box, row 7, column 2, row 8, column 2, and row 8, column 3 can only hold 4, 6, and 7, so those digits must go in those cells. No other cell in the bottom-left box can be 4, 6, or 7.
Look in a row, column or box for three cells whose candidates, taken together, are only three digits. Each cell need not have all three. Remove those digits from the region's other cells.
This example, step by step
Where to lookA pointer first: in the bottom-left box, 3 cells can only hold the same 3 digits.
Naked tripleIn the bottom-left box, row 7, column 2, row 8, column 2, and row 8, column 3 can only hold 4, 6, and 7, so those digits must go in those cells. No other cell in the bottom-left box can be 4, 6, or 7.
In hints
The hint marks the three cells and their digits, and strikes those digits from the region's other cells in orange.