瑜伽師地論略纂 11.162

11.162

論云「修無常想、苦想、無我想」者，此為三法假也。不言空者，即無我，同故不說。

When the treatise speaks of "cultivating the perception of impermanence, the perception of suffering, and the perception of non-self", these are designated designations based on the three dharmas. As for not mentioning emptiness: emptiness is identical to non-self, and so, being the same, it is not mentioned.