"Hidden triples" technique is very similar to "Hidden pairs" and works on the same concept.
"Hidden triples" applies when three cells in a row, column, or 3x3 block contain the same three Notes. These three cells also contain other candidates, which may be removed from them.
It will be easier to understand this technique if you look at the example.
Take a look at the highlighted cells. There are only three cells, which contain repeated numbers: 5, 6 and 7. This means each of these numbers must occupy one of these cells. And any other numbers cannot be found here. If so, 5,6 and 7 cannot be presented in any other cell of this 3x3 block as well."
After this conclusion, extra numbers can be deleted from the Notes to avoid confusion.
That is how "Hidden Triples" technique works while solving Sudoku.