Benjamin Sommer explains the trito-Isaiah theory.
Benjamin Sommer, "Appendix: Was There a Trito-Isaiah?" A Prophet Reads Scripture (Stanford: Stanford University Press, 1998), 187-195
Since the late nineteenth century, most biblical scholars have separated Isaiah 40-66 into two parts. These scholars view the last eleven chapters of the book as the work either of a post-exilic writer who lived as much as a century after Deutero-Isaiah or as the work of a group of writers, some of whom lived shortly after the exile and others of whom lived more than a century later. These scholars point to several features of the latter chapters that suggest the presence of a new author. Chapters 40-55 contain little prophetic rebuke and emphasizes consolation and exuberant promises of restoration. In chapters 56-66 on the other hand, religious and social criticism appear with greater frequency (e.g., 57.3-13, 58.1-12, 59.1-8, 63.7-64.12). These prophecies differ form the preceding one not only in tone but in literary style: while chapters 40-55 contain mostly poetry, chapters 56-66 mix prose compositions with poetry. Furthermore, unlike chapters 40 and following, the latter chapters seem to stem from the post-exilic period. Isa 56.8 ("I shall gather in again in addition to those gathered in") indicates that at least a partial ingathering of exiles has already taken place. A sense of disappointment pervades some of these chapters, and this might be attributed to the failure, after the exile had ended, of Deutero-Isaiah's more spectacular prophecies (e.g., predictions concerning the exalted position of the restored Jerusalem, the obeisance of foreign kings, the universal recognition of YHWH). References to a temple in these chapters (56.7, 60.7, 62.9, 65.11, 66.6) lead some scholars to assert that they must have been composed after the rebuilding of the Temple in 516 B.C.E, though none of these verse actually describes the Temple as already standing. More persuasive evidence of a post-exilic dating occurs in the interest these chapters display in issues that concerned other authors in that age: the inclusion of foreigners in the Judean community (Isa 56.1-7; cf. Ezra 9.1-4, Neh 9.2); the nature of the priesthood (Isa 61.5; cf. Ezekiel 44); the cleavage between different factions (Isa 57.19-21, 65.13; cf. Ezra 4.1-5). These distinctive characteristics are not evenly distributed throughout 56-66, and chapters 60-62 especially resemble 40-55 in tone and style. But these features lead most biblical scholars to posit separate authorship for chapters 56-66.