I've seen quite a few people sort of freaking out about Blaine Gabbert being the "3rd quarterback" on the depth chart for the Tennessee Titans, even an article in USA Today saying that Jack Del Rio "announced" Luke McCown was the number two quarterback, which is false. Del Rio announced nothing and was directly asked if McCown was the number two, which he responded "No, we have two back up quarterbacks. David is the starter and we have two back up quarterbacks."
Now that being said, it's highly likely McCown opens up the first few weeks as the primary back up, which is not a surprise at all. Given the way to offensive line has played, it makes sense for Del Rio to not want his rookie quarterback out there and risk a major injury, especially if they are committed to having him sit. If Garrard is nicked early, McCown can step right in and pick up the offense. Not only that, but the 3rd quarterback designation doesn't exist anymore, so it really doesn't matter. The quarterback depth chart is more akin to an MLB bullpen now than 2nd and 3rd string.