The “3,000 existences contained in one thought” (ichinen sanzen) doctrine was first expounded by Grand Master T’ien-t’ai in his Great Concentration and Insight (Mo-ho chih-kuan), fascicle 5, (chapter 7). According to him:

“A mind by nature contains ten realms of living beings (realms of hells, hungry spirits, beasts, asura demons, men, gods, śrāvaka, pratyekabuddha, bodhisattvas, and Buddhas). Since these ten realms contain one another, there exist 100 realms in one mind. Each of these 100 realms, furthermore, consists of ‘three factors,’ that is to say, living beings, the land on which they live, and the five elements of living beings (matter, perception, conception, volition, and consciousness). It also possesses “ten aspects” (form, nature, substance, function, action, cause, condition, effect, reward, and ultimate equality of these aspects). Thus, 30 modes of existence are in one realm and 3,000 modes of existence in 100 realms. In short, 3,000 modes of existence are contained in a mind at any given moment. When there is mind, even for a momentary flash, 3,000 existences are in it. … Thus a mind is unfathomable.”

Kanjin Honzon-shō, A Treatise Revealing the Spiritual Contemplation and the Most Verable One, Writings of Nichiren Shōnin, Doctrine 2, Page 127