Hidden Triple

A hidden triple occurs when, in a house (row, column, or box), three digits can appear in only three cells, while those three cells still contain other candidates. In that case, all other candidates can be eliminated from those three cells. No matter how the digits are arranged, they must occupy those three cells.

Hidden Triple is a size-$3$ subset.

A hidden triple can appear in a row. For example, in row $7$ below, digits $4$, $7$, and $9$ can appear only in $R7C2$, $R7C3$, and $R7C7$, so all other candidates can be eliminated from those three cells.

A hidden triple can also appear in a column. For example, in column $5$ below, digits $1$, $3$, and $7$ can appear only in $R4C5$, $R6C5$, and $R8C5$, so all other candidates can be eliminated from those three cells.

A hidden triple can also appear in a box. For example, in box $3$ below, digits $1$, $2$, and $8$ can appear only in $R1C7$, $R1C8$, and $R2C8$, so all other candidates can be eliminated from those three cells.