俱舍論頌疏論本 3.43

3.43

釋曰：唯無漏後三者，謂未知當知、已知、具知根，此之三根最居後故，名為後三，體唯無漏。

The commentary explains: "Only the last three are untainted" refers to the faculties of one who will come to know what is not yet known, one who has come to know, and one who possesses complete knowledge. Because these three faculties are placed at the very end, they are called the last three; their essential nature is exclusively untainted.