As a Trinitarian Christian of the Protestant persuasion, I see great significance in the number three as it pertains to the Godhead. This represents divine fullness, totality, sufficiency and completeness in the being and work of God.
However, when it comes to maths, I’m a complete dunce. As a child I found the nine times table difficult to master, although I see the mathematical connection between three, nine and twenty seven. As for the completeness of the canon of New Testament scripture and the connection between the number of books and the number of authors, well, that’s a new one for me! Here is some information I found on the significance of the numbers 3, 4, 7, 9, and 40:
The number 3 is thought to be the number of divine fullness, completion and perfection: The Trinity consists of Father, Son, and Holy Spirit and disciples are baptised in the name of the Father, in the name of the Son, and in the name of the Holy Spirit (Matthew 28:19). Man is a threefold being with body, soul and spirit. The Bible speaks of many threefold blessings: Faith, hope and love; grace, peace and mercy; the grace of the Lord Jesus, the love of God, the communion of the Holy Spirit (2 Corinthians 13:13).
The number 4 is thought to be the number of creation: North, south, east and west; the four seasons; the four phases of the moon. Significantly, Revelation 4:6-8 describes four living creatures around the throne, giving unending praise, glory, honour and worship to him who sits on the throne.
The number 9 has significance inasmuch as Jesus was crucified on the third hour (9 a.m.), cried out to his father and then dismissed his spirit on the ninth hour (3 p.m.). For this reason the number 9 is sometimes viewed as the number of judgment and finality. The fruit of the Spirit has 9 elements that all work together (Galatians 5:22-23) and there are 9 gifts of the Spirit (1 Corinthians 12:8-10).
The number 40 is often understood as the “number of probation or trial” because it appears so often in contexts dealing with judgment or testing. For example: the Israelites wandered for 40 years (Deuteronomy 8:2-5); Moses was on the mount for 40 days (Exodus 24:18); 40 days were involved in the story of Jonah and Nineveh (Jonah 3:4); Jesus was tempted for 40 days (Matthew 4:2); Jesus was tempted for 40 days and 40 nights (Matthew 4:2) and there were 40 days between Jesus’ resurrection and ascension (Acts 1:3). Source: https://www.gotquestions.org/40-days-Bible.html
I was unable to find any useful information on the significance of the number 27, but the partial quote below shows that the New Testament concludes with the Revelation of Jesus Christ, the twenty-seventh book of the Protestant New Testament Christian canon:
In the Bible, the number 7 is identified with something being “finished” or “complete.” From Genesis chapter one that association continues, as the number 7 is often found in contexts involving completeness or divine perfection. In the book of Revelation the number 7 is used there more than fifty times in a variety of contexts: there are seven letters to seven churches in Asia and seven spirits before God’s throne (Revelation 1:4), seven golden lampstands (Revelation 1:12), seven stars in Christ’s right hand (Revelation 1:16), seven seals of God’s judgment (Revelation 5:1), seven angels with seven trumpets (Revelation 8:2), etc. In all likelihood, the number 7 again represents completeness or totality: the seven churches represent the completeness of the body of Christ, the seven seals on the scroll represent the fullness of God’s punishment of a sinful earth, and so on. And the book of Revelation itself is the capstone of God’s Word to man. With the book of Revelation, the Word was complete (Revelation 22:18). https://www.gotquestions.org/number-7-seven.html
The 39 books of the Old Testament point to the coming of the Christ, the Messiah, and yes, the canon of Holy Scripture is completed with the 27th book of the New Testament.