[I]t is debatable whether Chih-i ever conceived the idea of one single Buddha, or found it meaningful. There is, in fact, a fundamental difference between the doctrine that “the three bodies are one body” and the idea that “all buddhas are one Buddha only” (issaibutsu ichibutsu), which would later be put forward in Japanese Tendai. Chih-i acknowledged, and justified, the existence of other Buddhas, and did not eventually reduce them to Śākyamuni Buddha (they are not Śākyamuni’s upāya). In the last analysis, Chih-i regarded Śākyamuni only as the most important Buddha of the Lotus Sutra and only as the Buddha of the present world. He claimed that the three bodies all reveal the “origin,” but he never qualified this original time as the absolute time. His “origin” is just the archetypal movement, the attainment of buddhahood.
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 229

[Chih-i’s] integration [dharma body, the recompense body and the transformation body] notwithstanding, Chih-i eventually puts the accent on one of the three bodies:

“[Chapter 16 of the Lotus Sutra] reveals the three bodies. If they are differentiated in a vertical sense, the true one is the recompense body. The wisdom of the recompense body, being one with what is above and in accord with what is below, encompasses the three bodies. … The text says ‘In the very far distant past since I became Buddha, I have benefited human beings in the three worlds.’ What is enlightened is the dharma body, what causes enlightenment is the recompense body. Because the dharma [body] and the recompense [body] become one, things may receive benefits. … Thus, the correct meaning [of the scriptural passage] is to postulate the virtues of the Buddha in his recompense body.”

This is perhaps the most interesting feature of Chih-i’s theory of the three bodies. The saṃbhogakāya represents a Buddha who has a beginning, and thus is finite before attaining enlightenment, but who becomes immeasurable, infinite, after his awakening. It exemplifies a Buddha who encompasses in himself both historical existence and universal principle: not an absolute Tathāgata who assumes for some time a phenomenal form and then goes back to his true nature, but a Tathāgata who is, at the same time, his true nature and his temporal manifestation.
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 227

Chih-i’s exegesis of the sixteenth chapter of the Lotus Sutra in his Fa-hua wen-chu contains a criticism of previous interpretations of the meaning of “distant past,” and a discussion of different categorizations of the Tathāgata: the twofold Buddha-body and the threefold Buddha-body, the Buddha of the origin, and the Buddha of the trace. Here I will examine how Chih-i applies the theory of the threefold Buddha-body to the Buddha of the Lotus Sutra. The scripture does not mention the different bodies of the Buddha but Chih-i employs existent theories of Buddha-bodies to illustrate the meaning of the text and, at the same time, to present his solution to the conflict between a noumenal and a phenomenal Buddha.

The three bodies the Buddha is endowed with are the dharma body, the recompense body (saṃbhogakāya), and the transformation body (nirmāṇakāya). Chih-i explains the nature of each of these three bodies and the way in which their enlightenment is displayed, supporting his arguments with passages from chapter 16.

The dharma body is defined as a principle “without causes and without results, whether there is a Buddha or no Buddha, everywhere present but without difference, without movement and yet coming forth [i.e., enlightened].” Chih-i infers this from the sentence in the Lotus Sutra: “There is neither birth nor death, or going away or coming forth; neither living nor dead, neither reality nor unreality; neither thus nor otherwise.” The dharma body is therefore a principle which reveals the perfect suchness without distinctions. Its enlightenment is the unchangeable, pure-by-nature tathāgatagarbha (that is, the buddha-nature), which allows the Tathāgata to “know and see the aspect of the triple world as it is, in its real nature.” Since the dharma body is in accord with the principle of suchness, both its nature and its appearance are eternally as they are, whether it is manifested or not as a Buddha; therefore it is not relevant whether it is measurable or not, that is, whether it has duration or not. In another commentary on the Lotus Sutra, Chih-i refers the phrase “neither thus nor otherwise” to the Middle Way, which in Tendai philosophy is synonymous with the real truth.

The recompense body has its scriptural evidence in the passage which proclaims: “The power of my wisdom is such, the light of my wisdom shines infinitely, my life is of countless kalpas, from long-cultivated karma obtained.” Chih-i explains that wisdom (the Buddha-eyes) is the foundation of this aspect of the Tathāgata: it is through wisdom that the Tathāgata attains buddhahood, it is wisdom that allows the recompense body to partake of the ultimate reality. As we shall see, here the emphasis is on the practice which leads to buddhahood.

The third body, the nirmāṇakāya, is characterized by ever-changing form and colors, and by its continuous appearing in the world. This is the meaning of the passages in the sutra “…whether I show myself or others, my deeds or other’s,” and “…revealing myself extinct and not extinct.” The nirmāṇakāya appears in numerous lives and numerous extinctions, is endowed with names which are never the same, and has different ages (the Buddha gives different accounts of the duration of his life). The Tathāgata in this aspect attains enlightenment in a particular place, as shown by the scriptural assertion that “Śākyamuni Buddha left the palace of the Śākyas and entered the place of enlightenment, not far from the city of Gayā.” The life of the nirmāṇakāya is affected by the principle of causation. Being bound to causality, this body is measurable; hence it typifies Śākyamuni as a Buddha restricted in both temporal and spatial terms. Yet, Chih-i underlines the idea that, because finite impermanence cannot be the principle that informs the existence of a Buddha, the transformation body can be seen as partaking in the immeasurable if one does not speak of its activity.

According to Chih-i, in fact, the three bodies are both permanent and impermanent, and are all three inherent in the Buddha of the Lotus Sutra: “One body is three bodies; it is not one, it is not different.” Chih-i here employs the point of view of the “perfect teaching” and applies the principle of “one is three,” which characterizes this type of teaching, to the three Tathāgatas, thus introducing a perspective quite different from that of earlier interpretations. He calls the virtue of being neither one nor three a “secret” or “mysterious” quality and presents it as peculiar to the Buddha of the Lotus Sutra, which other scriptures do not reveal. He denies that the three bodies are either in a horizontal, that is, equal, relation (referring to their innate merits) or in a vertical, that is, hierarchical, relation (referring to the merits derived from practice).
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 226-228