。
Mother divided the cake into three pieces.
Literal
Mother [topic-は] cake [obj-を] three [into-に] divided.
三つ is the native Japanese counter for general objects (ひとつ, ふたつ, みっつ...) — used up to 十, distinct from the Sino-Japanese 一, 二, 三 + specific counters. に marks the result or endpoint of the division: 'into three.' 分ける takes this に-result construction naturally.