I always thought Matthew, Mark and Luke are different books originally were written by different people. But recently I was told that Matthew, Mark and Luke are originally the same book but in different versions. At the beginning there was one book only and then it was edited or adjusted by different people so at the end we left with three versions of the same book. Is it true?
Matthew, Mark, and Luke, aka the Synoptic Gospels, share common elements and parallels and were written by different people for different audiences. Your question has been (and still is) debated among Biblical scholars. My answer is, at best, a brief summary with some pointers to references that may be helpful and some encouragement for you.
Mark is believed by most scholars to be the earliest written account. There is evidence to suggest that Matthew and Luke borrowed from Mark. There is also evidence for a lost book of Jesus' sayings, known as Q, that influenced Matthew and Luke.
To say that they are different versions of the same original book does not address the historical origins of ancient manuscripts, oral traditions, the historical contexts for which they were written, and the editorial practices that created the manuscripts from which our modern translations were (and are) derived.
What you were told--that there was one original book--may be an attempt to support Biblical literalism and inerrancy and to say that there once was one original and perfect manuscript. This view is more commonly held within American evangelicalism.
Each Gospel stands on its own and has profound lessons for Christians. The study of scripture can be a rich and fulfilling activity.
A useful reference in studying the Synoptic Gospels, specifically the parallels among them, is "Gospel Parallels: a Comparison of the Synoptic Gospels" by Burton H. Throckmorton. https://www.amazon.com/dp/0840774842/
Another book that covers the doctrines of Christian scripture is "Inspiration and Authority: Nature and Function of Christian Scripture" by Paul Achtemeier. https://www.amazon.com/dp/0801045428/
For deeper study, the "New Interpreter's Bible Commentary" in 10 volumes is an excellent resource. https://www.amazon.com//dp/1426739125/
Three distinctly different men each wrote their own gospel account of the life, death and resurrection of Jesus Christ. While many scholars think that Mark’s gospel was written first and that Matthew and Luke copied from it, this claim has been challenged. There is evidence to support the view that Matthew wrote his gospel account first:
Historian and papyrologist Carsten Thiede proved (in 1994) that a papyrus kept in Oxford's Magdalen College was much younger than previously thought. He dated it to AD 65 at the latest, and possibly earlier than that. This papyrus was of the Gospel of Matthew, relating events on the last night before Jesus died. It turns out to be the oldest remains of a Christian book ever found.
In our time there's been a rise in claims that the biblical Gospels were not historical, eye-witness accounts but just unreliable folk-lore; stories about Jesus collected long after his death, cobbled together to meet the needs of a persecuted little band of Christians. Scholars tended to describe the Gospels as "community creations" - myths designed by a committee rather than vivid biographies. In 1993 a seminar of 100 experts declared that only 20% of the sayings attributed to Jesus in the Gospels were authentic or near approximations.
The Magdalene Papyrus proves that the Gospel of Matthew is authentic and was written within 30 years of Jesus' death, and written by those who had witnessed it. Matthew's account was circulating amongst the same generation of people who walked and talked with Jesus. Significantly, that AD 65 manuscript reads EXACTLY the same as modern translations of Matthew’s Gospel. Source: The Jesus Papyrus by Carsten Peter Thiede and Matthew d’Ancona (Weidenfeld & Nicolson 1996)
In his book Why Four Gospels? Dr. David Alan Black asserts that Matthew was the gospel written first. According to Black, the gospels, arranged in order from earliest to latest, are Matthew—Luke—Mark—John. The following points are a summary of Black’s view of this chronology:
• After Christ was resurrected from the dead and the Church was born (Acts 2), the believers realized the need for a written record of the account of Christ’s life. Matthew was selected for the task of producing that account, and, under the inspiration of the Holy Spirit, he penned his work before the Jerusalem believers were scattered from the persecution of Herod Agrippa I, which occurred in AD 42.
• Paul began his evangelistic work and found the need for a gospel account that spoke more to the Gentiles (Matthew’s gospel was written for a Jewish audience). Paul worked with Luke to produce his gospel, which was completed sometime between AD 58 and 60.
• During Paul’s detention in Rome (AD 60—62), he asked Peter to personally authorize Luke’s gospel, which he did. While in Rome, Peter delivered a number of testimonies about Christ’s life to an illustrious Roman audience. Those messages were recorded by John Mark, Peter’s secretary. After Peter was martyred, Mark’s work was published as a gospel in AD 66 or 67.
• John’s gospel was then written later and published while John lived in Ephesus. The completion of the Gospel of John rounded out the eyewitness accounts of Jesus’ life.
Support for Black’s timeline is found in the writings of the early church fathers. Referring to the Gospel of Matthew, Origen (AD 185—254) writes, “The first written was that according to the one-time tax collector but later apostle of Jesus Christ, Matthew, who published it for the believers from Judaism.” Clement of Alexandria (AD 150—215), quoted by Eusebius, supports Origen and provides some details for Luke: “Clement has set down a tradition of the earliest elders about the order of the Gospels, and it has this form. He used to say that the earliest written gospels were those containing the genealogies [Matthew and Luke].” Clement also speaks of Mark when he writes, “Peter was publicly preaching the gospel at Rome in the presence of some of Caesar’s knights and uttering many testimonies about Christ, [and] on their asking him to let them have a record of the things that had been said, wrote the gospel that is called the Gospel of Mark from the things said by Peter.”
While it is impossible to be certain about the order in which the gospels were written, the prevailing viewpoint seems to be that the order was Matthew, Luke, Mark, John. https://www.gotquestions.org/which-gospel-written-first.html
It is possible that, whichever Gospel was written first, the other Gospel writers had access to it. That does not mean that Matthew, Mark and Luke are simply different versions of one book. It means that Matthew wrote his account (aimed specifically at a Jewish audience), Mark wrote his account (aimed specifically at non-Jews, or Gentiles) based on information from the apostle Peter, and Luke wrote his account after careful examination of eyewitness accounts:
Many have undertaken to draw up an account of the things that have been fulfilled among us, just as they were handed down to us by those who from the first were eyewitnesses and servants of the word. Therefore, since I myself have carefully investigated everything from the beginning, it seemed good also to me to write an orderly account for you, most excellent Theophilus, so that you may know the certainty of the things you have been taught (Luke 1:1-4).
As for the suggestion that Matthew and Luke were influenced by what is known as the “Q” document, be aware that no portion or fragment of such a document has ever been discovered. None of the early church fathers ever mentioned a Gospel “source” in their writings. “Q” is the invention of liberal “scholars” who deny the inspiration of the Bible. They believe the Bible to be nothing more than a work of literature, subject to the same criticism given to other works of literature. Again, there is no evidence whatsoever for a “Q” document—biblically, theologically, or historically. Source: https://www.gotquestions.org/Q-Gospel.html
To conclude, Matthew, Mark and Luke are different books originally written by different people. You only have to examine the context and styles of writing to establish that. There is no evidence to suggest that the first gospel written (probably Matthew’s gospel and not Mark’s) was edited and changed to produce three versions of the first, original book.