釋氏要覽 2.198

2.198

謂於一真如上。義說為三也。一真如。是覺性。名佛寶。二真如。有執持義。名法寶。三真如。有和合義。名僧寶。

This means that upon the single tathatā, they are explained in meaning as three. 1. Tathatā as the nature of awakening is called the Buddha Jewel. 2. Tathatā having the meaning of upholding is called the Dharma Jewel. 3. Tathatā having the meaning of harmony is called the Saṅgha Jewel.