noun
division into three; one third
Refers to splitting something into three equal parts, or one of those parts. Often used in contexts like dividing a sum, a space, or a task.
費用を三つ割りにして、一人一万円ずつ払った。
We split the cost into three and each paid 10,000 yen.
ケーキを三つ割りにして、みんなで食べた。
We divided the cake into three and ate it together.
三等分 is a more formal or mathematical term for dividing into three equal parts, while 三つ割り is a more everyday expression.
Compound of 三つ (mittsu, 'three') + 割り (wari, 'division, split'). The reading みつわり reflects rendaku voicing of わり.