成唯識論述記 1.423

1.423

述曰：此中遮總合成一相，彼宗自許三體相別故。立量云：三事和合所成之相，亦應有三；許即三體故；如體。

The Commentary states: Here, it refutes the joint composition into a single characteristic, because their school itself accepts that the three essences and characteristics are distinct. The syllogism is established: The characteristic established by the combination of the three entities should also be three; because you accept that it is identical to the three essences; like the essences.