I was a Christian once, but if someone were to take the Bible seriously, I'm sure they would know if God was planning to release a Bible 3.0; Newest Testament. If something like that were coming, I'm sure God would have taken a few steps to alert people ahead of time so they won't fall into a trap.
Merely making your own third part of the Bible isn't enough, you'd have to showcase evidence that you were really told to do it in order for me to believe it. If you're a talented writer you might snag a few people, but any devout follower of a deity would believe that they would be notified of any new "updates" to their holy book.