A first epistle to Corinth is referenced at 1 Corinthians 5:9. St. Paul also makes reference to an epistle to the Laodiceans in Colossians 4:16. These works are known as "The Lost Epistles of Paul." There is also a third epistle to the Corinthians that was purportedly canonical in the Armenian Church to which we are in communion. How could God allow these works to be lost? Would we include them in the Bible if they were still in our possession? Why do we not we regard 3 Corinthians as canonical?
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.