The Naked Triples method can be used if three cells in the same row, column, or box collectively have only three candidates. Those three cells must therefore contain these three numbers in some permutation, so any other unfilled cells in this row, column, or box cannot contain any of these three numbers. Therefore, these three numbers can be eliminated as a candidate for those unfilled cells.
Using the Naked Triples method, which number goes in cell B6?