大般涅槃經義記 4.230

4.230

迦葉言下問答重顯。迦葉先問是義云何，下佛釋之。釋意如何？明無量劫乃有一佛出現於世，由佛難出，為化眾生權開三乘而度脫之，非實有三，所以言無。

From "Kāśyapa said" downward, the question and answer clarify it once again. Kāśyapa first asked what this meaning was, and below the Buddha explains it. What is the intention of the explanation? It clarifies that only once in immeasurable kalpas does a buddha appear in the world; because a buddha is rare to appear, in order to transform sentient beings, he expediently opens the three vehicles to cross them over and liberate them; they are not genuinely three, which is why he says they do not exist.