Division into three (equal) parts; to divide something into three parts (can be used with the verb suru).
ケーキを三等分して、それぞれに果物を乗せた。
I cut the cake into three pieces and put fruit on each.
この問題は三等分して考えたほうが分かりやすい。
It's easier to understand this problem if you consider it in three parts.
地図を三等分して、担当区域を決めた。
We divided the map into three parts and assigned responsibility areas.