俱舍論頌疏論本 23.44

23.44

言三餘者，餘謂獨覺，在佛乘外，故名為餘。聲聞種姓煗、頂、忍三容可轉成獨覺乘故，名為三餘。

Regarding "with three the other": "the other" refers to the pratyekabuddha; being outside the Buddha vehicle, it is named "the other." Because the three of Heat, Summits, and Acceptance of the śrāvaka lineage can all be transferred to become the pratyekabuddha vehicle, it is called "with three the other."