If we have a "Third Testament" in some Christian religion, it won't have the Book of Revelation in it, and this is in large part because of Revelation 22:18, which is a pretty strong condemnation against adding things to the Bible. Revelation was historically questioned since before the New Testament canon was solidified, and questioned again by people like Martin Luther.
Although any Christian sect which adopts a "Third Testament" will likely have alterations to the first and second Testaments as well, assuming they even have it to begin with. If Gnostics can be considered Christians, I'd expect a Gnostic group which compiles a Biblical canon to omit a huge part of the Old Testament, if not cut it out entirely. Other groups might recognise different apocrypha as canon. The odds of a Bible with three testaments being the same as a standard Bible with a bunch tacked on is unlikely, although if a Third Testament emerges from a Protestant sect, you might be able to get something like that (as the Mormons do with their creative interpretation of Revelation 22:18).