How it works
Three cells in the same row, column or box carry only three candidate digits between them, combined — not necessarily three each. Those three digits must fill those three cells between them, so every other cell in that unit can have them eliminated.
Try it
Look at column 6. Let's pencil in what's still possible for Row 2, Column 6, Row 4, Column 6, and Row 6, Column 6, then see why 2, 5, and 8 are locked into just those three cells.
Tap each of Row 2, Column 6, Row 4, Column 6, and Row 6, Column 6, then tap every digit that's still possible there — check what's already used in that cell's row, column and box. Press "Check my work" when you're ready.
Nice — every candidate for Row 2, Column 6, Row 4, Column 6, and Row 6, Column 6 is confirmed.
Not quite — look again at what's already placed in each cell's row, column and box, then check again.
Row 2, Column 6, Row 4, Column 6, and Row 6, Column 6 carry only 2, 5, and 8 between them, combined — not necessarily one each. Whichever cell takes which digit, all three are used up right there — so none of them can appear anywhere else in column 6.
Which means 2, 5, and 8 can be crossed out everywhere else in column 6.
Tap a highlighted cell, then tap its red digit on the pad below to cross it out — the highlighted cells show why it can't go there.
That's it — nicely done.