The answer
The Third Epistle to the Corinthians is considered to be pseudographical, meaning that the author is not authenticated. There is no reason to incorporate this manuscript into our canonical books because of its lack of authenticity. In addition, there was much controversy in the early church regarding the misuse by Gnostics whose heretical teachings sought support for their ideology by misinterpretations of various texts. To combat these heresies, some texts could have been doctored, such as this one. Thus, the Church has been protective from inclusion of any potentially altered manuscripts.Answered by His Eminence Metropolitan Youssef (opens in a new tab)