金光明經文句記 3.188

3.188

二、「此」下，結示。即鼓之三德，對光秖是所照法身，即此法身攝報攝應為上三德，故云觀一而見三佛也。

Second, from 'this' onward, concluding the demonstration. The three virtues of the drum, relative to the light, are simply the illuminated Dharma body; this very Dharma body subsumes the reward [body] and response [body] as the above three virtues; hence it says that contemplating one, one sees the three Buddhas.