These can be harder to spot than naked pairs but the principle is essentially the same. Again it applies to rows, columns and subgrids but we will only consider one of those at a time.
You're looking for a triplet of candidate numbers used in three different cells within the same subgrid. As this trio of cells must contain all occurences of the tripled digits (but we don't know which order just yet), we can eliminate any other uses of the digits in the subgrid.
This technique is also known as locked candidates and is essentially a variation of the pointing and claiming techniques, but with three digits at once.
N.B. You should only use this strategy once all cells in the subgrid (or row or column) have been filled with candidates.
In the example, the numbers 4, 5 and 8 comprise a triple. The center cell only shows candidates 4 and 5 but we can still use it in the triplet by imagining that 8 is also a candidate and because there are no other possible numbers for the cell. We can now eliminate any other uses of the digits 4, 5 or 8 elsewhere in the subgrid.