The technique works on the principle of "Obvious Pairs". But in this case, in order to solve the Sudoku, you need to find 3 squares, in which we must have the same numbers.
The method only seems complicated: in practice it is easy to work with. Let's look at the following example. The numbers missing from the line are 1, 2, 6, 7, 9. By the "last possible number" method we understand that in the middle 3x3 block there will be a number 1, and in the 3x3 right block there can only be a number 7. So we can cross out these numbers from the bottom left row in the left block.
We have already eliminated numbers 1 and 7 from the candidates. In the row of the left block, the candidate numbers remain: 2, 6 and 9. These will be our "obvious triples".
Now look at the left 3x3 block. Only 2, 6, or 9 can be placed in the bottom three cells, and no other digits can be placed (according to Sudoku rules). So we can remove these numbers from the rest of the cells - we remove the nines (there are no more twos and sixes in this block). In the middle left cell we have "Obvious singleton" - 1. That's what we write.
Next, we remove the 1 from all the notes and add the numbers - 3 and 9.
The "Obvious triples" method allows you to narrow your search and minimize the risk of errors due to duplicate values.
Once you have successfully mastered this method and learned how to solve simple Sudoku, you can move on to more difficult puzzles and explore other strategies.