俱舍論疏 21.128

21.128

論「謂欲界繫至立三遍知」，答也。謂欲界見苦、見集合一遍知，見滅、道各一遍知，合成三也。

The treatise says: "Namely, those bound to the desire realm […] establish three comprehensions": this is the answer. It means that what is abandoned through seeing suffering and seeing origination in the desire realm are combined into one comprehension, while seeing cessation and seeing the path are each one comprehension, combining to make three.