秦至西漢本以丞相為宰相，到東漢演變成地位相等的三公並列宰相。西漢晚期至新朝是從獨相制變化為多相制的關鍵時期，本文旨在釐清三公官制成立的過程。西漢晚期的兩次三公官改制，目標是以中朝的大司馬與外朝的丞相、御史大夫並列宰相。將大司馬改造成和外朝的丞相具有相同職能的職官，是三公官改制的關鍵。大司馬本來是一個頭銜，由輔政的將軍兼任，沒有印綬、官屬。經歷兩次改制後，大司馬獲得印綬、官屬，得到宰相的辦事機構及監督百官執行權，也要承擔調和陰陽、安輯百姓的責任。在選任上，大司馬本來常以任近侍官的外戚擔任，選任丞相、御史大夫則重視循序漸進的任官資歷。改制後，過去任官資歷足以擔任丞相的一般官吏也能任大司馬。儘管大司馬經過兩次改制逐漸「外朝化」，它始終未失去在宮中「領尚書事」輔政的權力，與另外二公並非同質的職官。直到王莽建立新朝時，大司馬才失去輔政權，完成外朝化，三公也確立分職，改制遂告完成。不過三公官似乎不能滿足王莽對宰輔的想像，他以古制為基礎，另設高於三公的四輔。四輔不但與三公一起管理全國政事，也在宮中輔政，為兼攬內、外的宰相。東漢僅繼承三公官，兼治內外的四輔則未被採用。

In the periods of Qin and Western Han dynasties, Imperial Chancellors (chengxiang 丞相) served as chief aides to the emperor – zaixiang (宰相), a role later ceded to the Three Excellencies (san gong 三公) by the time of the Eastern Han. Between the end of the Western Han, and the establishment of the Xin Dynasty, a pivotal change occurred in the role of the chief aide – it went from being held by a single chancellor, to a collective function. This study aims to expound the formation of the latter, embodied in the system of Three Excellencies. The nature of the Three Excellencies was reformed twice in the final years of the Western Han, with the aim of endowing the Marshal of State (dasima 大司馬) in the Inner Court alongside the Imperial Chancellor (chengxiang 丞相) and the Imperial Counsellor (yushi dafu 御史大夫) of the Outer Court with the function of zaixiang. A key change in the process of reforming the system of Three Excellencies was making the functions of Marshal of State identical to those of Imperial Chancellor. Marshal of State was originally an official title given to generals tasked with assisting the ruler, but individuals holding it were not given imperial seals, or subordinate bureaucrats. After two reforms, Marshals of State received imperial seals, and subordinates, and were given administrative, and supervisory powers of zaixiang, as well as tasked with maintaining a cosmic balance, and peace among the populace. Originally, the individuals selected for the position of Marshal of State were often distaff relatives of personal imperial attendants, while those tapped for positions of Imperial Chancellor, and Imperial Counsellor were qualified public officials advancing through the ranks. After reforms, officials qualified to hold the post of Imperial Chancellor could also become Marshals of State. Although the responsibilities of Marshal of State have gradually moved from the Inner to the Outer Court following the two reforms, this position never entirely lost its ‘internal’ prerogatives of the Manager of the Affairs of the Masters of Writing (ling shangshushi 領尚書事), which set it apart from the other two Excellencies. Only with the establishment of Xin Dynasty by Wang Mang did the Marshals of the State lose their function of assisting the ruler, and their prerogatives were moved entirely to the Outer Court, which coincided with a clear separation of responsibilities between the Three Excellencies. The process of systemic reform was thus complete. However, the Three Excellencies did not seem to fulfill Wang Mang’s expectations for chief aides. The new emperor based the mode of his rule on ancient examples, and established the offices of Four Advisers (sifu 四輔), superior to the Three Excellencies. The new group held the reins of state administration alongside the Three Excellencies, on top of assisting the emperor as zaixiang, thus combining Inner, and Outer Court functions. The ensuing Eastern Han Dynasty continued the appointment of the Three Excellencies, but abandoned the combined internal-external office of Four Advisers.