John is accounted in his own book, since he was there, referring to himself as 'the one Jesus loved' or 'beloved'.
Matthew, again, was one of the disciples, so was definitely an eye-witness.
Mark was Jesus's brother and became a follower of him, probably before his death, so he would have had his own and the accounts of Jesus's followers as a direct source. It's the historical equivalent of our modern day documentaries and historical accounts.
I mean, these books are different in the same way that different history books and documentaries are different today! And they are so similar to each other with so few contradictions that it can definitely be considered reliable by modern standards. The real argument is whether or not they were authored by who they say they were, which as I stated above, would be pretty hard to do since it's generally accepted that all of these books were written within 100 years of Jesus's death.