There are four canonical gospels of the New Testament: Matthew, Mark, and Luke (synoptic gospels) and then the gospel according to John. They all tell different accounts of the birth, life, death, and the ressurection of Christ, God incarnate. These writings all have unique elements but many have what is called “Gospel Harmony,” a word for word similarity between two or more gospels. When three gospels line up, this can be referred to as “Triple Tradition,” i.e. a three way redundancy which exists primarily in the synoptic gospels. It has been under much debate as to how the “Q” source developed and for good reason, the interconnectedness of the gospels in a way determines the credibility of them as historical documents.
The actually act of figuring out the connectedness of the gospels is quite complicated, thousands of hours of parallel comparisons, and then the contexts of each must be compared to ensure that those comparisons actually have theological weight. Also, there are some linguistic differences that Biblical scholars have to look out for considering translations frequently have elements of speech from different time periods.