I don't think 3 at the back is in Pep's calculus, as he wants flying full backs (midfielders in disguise) and the extra defender costs him an attacker.
More likely, we have a deep lying midfielder who slots into the middle of the back four to pick up the ball for distribution, as the full backs push forward. That allows a few things to happen...wide two of three attackers to tuck in, the remaining two midfielders to roam behind the front 3/5, just as those full backs can also now become the wide midfielders, allowing the wingers to get down the wing.
The fluidity of Pep's formation requires two main things: the ability to retain the ball when we are exposed in our attacking formation, and the ability of the non-defenders (everyone except the center backs and DM) to move freely in and out of understood formations that do not always directly translate to their "position" on the field, but rather are based on where other players have found themselves.
To do both of those things requires special players. We have a handful of them already (Merlin, KDB, Yaya) and Pep is trying to teach the others, and must have already seen this ability (or the ability to adapt to him) in this summers targets, because this season is about not messing around, about having quality replacements fo injuries, late game fatigue or lack of performance, and to be able to rotate players without any noticeable drop off in quality or performance.
There has always been this notion that teams require the squad to have two full teams and a few spares. This may be the first season where we actually witness a squad that literally contains two first teams that can compete with any team, on any given day, with almost zero overlap of personnel...especially in the key midfield area.
The future is Blue!
P.S. All that said, he may well have seen Chelsea's success and decided that Walker and Mendy can operate as actual midfielders allowing his middle two and forward three to operate as a pack. Personally, I'd like to see us play the 5-3-2/3-5-2 and swarm teams with SEVEN attackers and defend with EIGHT defenders/midfielders. Can hardly wait to see how this all unfolds...!