Open box: 凵
凵 is called うけばこ, an open-box shape usually found around or below a character. Here it is a visual clue: 凸 is convex and 凹 is hollow, while 凶 means bad luck; do not assume the component gives all three the same meaning.
Write 凶 in 4 strokes; in 凸 and 凹, take care to keep the bottom open-box shape distinct (each character has 5 strokes).