To be fair this sounds like our tactics now.
One attacking full back. One defensive one.
No CDM, the only one that is is an exceptionally old player and needs to be replaced.
Yeah, it's a pretty normal thing. If we just cherry pick a handful of great teams: Abidal and Alves at Barcelona in their (1st) treble-winning team, Chivu and Maicon at Inter in their treble-winning team, Vogts and Breitner in Germany's '74 WC winning team, Howedes and Lahm in their WC winning team 30 years later...all of these players were primarily fullbacks, but the former group also played at centre back quite often, and the latter also played in midfield quite often. That's pretty much how they played at full back, one was additional centre back and one was an additional midfielder, because fullbacks are pretty much the only position on the pitch that need to occupy multiple roles in every game just by nature of covering the length of the pitch.
At the end of the day tactics in football is mostly just about outnumbering your opponents in the right places at the right times, so the general idea that you never want to leave your centre backs 2v2, and definitely not 3v2, is why you'll pretty much always have that third man. That's why back three's became so popular once attacking fullbacks came onto the scene: if you wanted both, you made them wing backs. That third man will either be in front of the centre backs or alongside them. Generally Sir Alex preferred having the centre mids playing with less restraint, so he was happy enough to have that 3rd centre back there as a covering fullback. He got lucky in a sense that Neville started as a centre back but as he got to his late 20s, he became pretty decent in the attacking phase too, so he was defence-first with a bit of quality. But when Neville retired and Evra showed he could carry the attacking threat single-handedly, he mostly preferred the likes of O'Shea, Brown, even Smelling and Jones to counter-balance him.