These three gospels are called the "Synoptic Gospels" because they are so similar in their content. These three gospels tell many of the same stories. In this way, they are very different from the Gospel of John. The word "synoptic" comes from Greek roots meaning "together" and "seen." These, then, are the Gospels that are "seen together" because they are so similar. Scholars have concluded that these three gospels are too similar to have been written independently. Instead, most scholars believe, the Gospel of Mark was written first and was then used (along with a lost source generally known as "Q") as a source by the authors of Matthew and Luke.
The best answer is wrong. The Gospels were not written together and have glaring inaccuracies. The worst inaccuracies are in the retelling of the resurrection. Only two women are mentioned as witnesses to the angel at JESUS's tomb in Matthew while three are in Mark. Mark tells about them bringing spices for the body, Matthew mentions nothing of it. In Mark the women are told to tell Peter and the discipes about the resurrection while in Matthew there is no command mentioned. In Mark only Mary Magdalene tells of the resurrection after she had fled in all out silent fear while in Matthew all the women were going to tell the disciples before JESUS interrupted them. These books were written by three different people and even the words they use, sentence structure etc. show this. I don't have the desire to show more differences in Luke as compared to the other two and this is just looking at one section of the books.
They are given this name because many of the parables and miracles are the same throughout them. However the four Gospels all represent Jesus in a different light.