Other than Jose Mourinho's
panic all-out-attack formation — you know, sub off Azpilicueta, throw on a striker, play Cahill, JT, Ivanović at the back — a three-man defensive line is something that probably hadn't been seen regularly in over two decades at Chelsea, if not longer.
Perhaps the most interesting part of Conte's implementation of the three-man defense is that he's doing it mostly with players not necessarily thought of as "suitable" for such a formation. David Luiz was of course born for the sweeper/libero role, which can exist in a three-man setup, but as it turns out that role doesn't exist in Conte's implementation.
Ahead of tonight's match at West Ham, Conte elaborated on his ideas behind setting up the new-look defense, detailing a few of the key attributes required of each position in the backline.
"The central defenders on the right and the left must be very fast and aggressive. The players that play in the middle of the three defenders must be more tactical, must reflect more and find the right position, and also call the line up and down."
It would appear that we have a ways to go to fulfill these requirements with an ideal setup. Gary Cahill, for example, is not exactly a defender I'd classify as aggressive (unlike, say, Kurt Zouma!), while having David Luiz deployed as the brains of the operation would've drawn guffaws just a month ago. (Azpilicueta actually is probably just fine then, though he loses a bit on the aerial side of things.) Then again, at Juventus, presumably a similar system worked quite well with the likes of Chiellini and Barzagli in the "fast" roles, so perhaps the requirements don't have to be filled exactly for things to work.
Having said that, it sounds like we're not going to try to shoehorn John Terry into a three-man defense tonight, with Conte hinting at the need to have a backup plan developed just in case we need it and using the occasion of the League Cup match to practice that in a real world situation.
"In these last three games we did very well. It's normal when you play with the same players. They take confidence in the role to play together."
"It's important also for example in this game, the cup, to find a new solution. If you have during the season an injury, then it's important to have a good solution ready."
-Antonio Conte; source: Chelsea FC
I'm guessing it'll be a 4-1-4-1, though maybe Conte has some more tricks up his sleeve.