首楞嚴義疏注經 6.346

6.346

諸行或對機不同，此三決定須說。又是決定成佛之因，佛佛皆爾，故云三決定義。

Although various practices may vary according to capacity, these three must definitively be taught. Furthermore, they are the definitive causes for attaining buddhahood, and so it is with every Buddha; therefore they are called "the three definitive meanings."