Hidden Triplets & Quads
As you may have guessed, Hidden Triplets work on the same concept as Hidden Pairs: When three given pencil marks appear in only three cells in any given row, column, or block, all other pencil marks may be removed from those cells.
In the example below, we see a block with only one cell solved and the rest of the cells pencil marked with all their possible candidates…
Now, if we study very closely, we can see that the three cells indicated in green (below) are the only cells that contain the pencil marks 2, 6 & 8.
Since 2, 6 & 8 are only possible in three cells, we can conclude that those three cells must be some combination of 2, 6, and 8! We can safely erase all the other pencil marks in them (erase the pink ones).
You may have noticed that the "8" pencil mark only appears in two of the cells. That's fine. The rule still fits. In fact, it gives us even more information later. The fact of the matter is that 2, 6, & 8, as a "set", only appear in three cells. Three numbers; three cells.
Hidden Triplets are kind of hard, huh? Well, the good news is this is the hardest of the "basic" techniques. And if you stopped here, you would still possess the skills to solve 95% of the Sudoku puzzles out there.
For the sake of being complete, there is one more technique I should mention real quick: Hidden Quads.
Similar to Hidden Pairs & Triplets, a Hidden Quad is when you find four different pencil marks inside only four cells in the same house. I should say, though, that these are extremely rare. It is quite possible that you will never even see one.
For this reason, I will not go too deeply into them. I'll just give you an example of one and leave it at that.
The puzzle below contains a "Hidden Quad". Can you find it? If you can't, don't be discouraged! In fact, if this is discouraging you at all, feel free to scroll down and click "Next", and you will not be any worse off for it. As I said, you can probably go your whole life without ever needing this technique.
But if you like a good challenge, here it is…
Hint: Hidden Quads only appear in houses with seven or more unsolved cells. The puzzle above has only two blocks, two rows, and three columns that meet that requirement.