Three cells in a unit form a naked triple when, between them, they hold only three distinct candidates. The cells do not need to show all three digits each — lists like 1-4, 4-9 and 1-9 qualify just as well as three full 1-4-9 cells. However the digits distribute, those three cells will consume those three digits, so the rest of the unit can be swept clean of them.
r3c2, r3c5 and r3c9 share the digit set {1,4,9} — those digits vanish from the rest of row 3.
Row 3 shows r3c2 with 1-4, r3c5 with 4-9 and r3c9 with 1-9. Three cells, and their combined candidates are just {1, 4, 9} — a naked triple in its broken form, no cell holding all three. The trio will absorb those digits however they fall, so the 4 pencilled at r3c4 and the 9 at r3c7 are removed from the rest of the row.
Four distinct digits across the three cells means no triple, full stop. And remember the count is over the union: 1-4, 4-9, 1-9 works precisely because the union stays at three.
These grids come from rated games on this site — our solver confirmed that naked triple was part of the shortest solving path for each one.
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