成唯識論述記 1.425

1.425

述曰：第七，比量也。汝言此三事和合共成一相之時，應不能成一；三體各別故，或前與後，體無別故；如不合時，相實有三，變合成一。彼計三事有不和合，即是未成大等法時，故得為喻。

The Commentary states: The seventh is an inference. When you say that these three entities combine to jointly establish a single characteristic, they should not be able to establish one; because the three essences are distinct, or because between before and after the essence is without distinction; like the time when they have not combined, the characteristics are really three, but through transformation and combination they become one. They posit that the three entities have a state of non-combination, which is when they have not yet established mahat and the other principles; hence this can serve as an example.