Regarding the threefold body of a Buddha, the Sūtra of Meditation on the Universal Sage Bodhisattva preaches:

“The three kinds of body which a Buddha possesses is expounded in Mahāyāna Buddhism. In particular, the Lotus Sūtra is equipped with all the teachings just as an ocean accepts all the waters. From this ocean (of the Lotus Sūtra) three pure bodies of a Buddha are born. These three bodies of a Buddha are the fields where human and heavenly beings can plant the merit of good deeds, and they are the most superior of accepting offerings from human and heavenly beings.”

The three bodies of a Buddha are (1) Dharma Body, (2) Reward Body, and (3) Accommodative Body. Each Buddha inevitably is equipped with these three. If we compare them to the moon, the Dharma Body is the moon’s body, the Reward Body is the moonlight, and the Accommodative Body is the shadow of the moon. As one body of the moon has three sides, each Buddha has the virtue of three Buddhas.

Shijō Kingo Shakabutsu Kuyō-ji, Opening the Eyes Service of Shijō Kingo’s Statue of Śākyamuni, Writings of Nichiren Shōnin, Volume 6, Followers I, Page 131