3-cell killer cage summing to 7

A 3-cell killer cage of 7 is fully locked: the only way to make 7 from 3 distinct digits is {1, 2, 4}. Whatever order they sit in, those digits are the cage's contents — every one of them eliminates from every cell of the cage's row, column, and box that lies outside the cage.

Locked digits: 1, 2, 4. Whichever combination is in play, these digits sit somewhere inside the cage — meaning the cage's shared row, column, and box can't have any of them anywhere outside the cage.