If 3 numbers only exist in three cells of a given unit while each cell contains at least 2 of these 3 numbers, then all other candidates can be removed from these 3 cells. For number set {2, 4, 5}, if any group of cells in same unit contains one of the combinations below is a Hidden Triplet:
{2, 4, 5, 8} {1, 2, 4, 5} {2, 3, 4, 5, 9}, or
{2, 4} {2, 3, 5} {4, 5, 7}, or
{4, 5} {2, 5, 8} {1, 2, 3, 4, 5}, or
{1, 2, 5} {2, 4, 8} {4, 5, 9}, or
...
To understand this technique, we will look at an example first:
In Row H, each number in number set {5, 8, 9} only occurs in [H1], [H3] and [H5]. Other cells in Row H do not contain any of these numbers. Therefore numbers 5, 8 and 9 must be placed in these cells and other candidates can be safely removed from these cells.
Below is another example:
In Column 7, no other cells except [F7], [G7] and [H7] contain candidates 3, 7 and 9. These three cells form a Hidden Triplet. Therefore, we can remove candidates 2 and 5 from [F7] and remove 5 from [H7].
Hidden Triplet can also be "hidden" in blocks:
In Block at [G7], candidates 3, 6 and 7 are found only at cells [G8], [G9] and [H8]. So we can remove 2 and 9 from [G8] and [H8] safely.
After applying Hidden Triplet Technique, a Hidden Triplet will become a Naked Triplet.
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