We could, but I would not like to see it. Would not be playing to our strengths for a start - three CBs and then one or two holding midfielders and you are already down to four or five players giving attacking options - if 2 are wing backs and 2 are strikers, that leaves just one max from midfield. And that doesn't even include accounting for the guy actually with the ball trying to make something happen. Not enough to break teams down in the modern game.
As it is now, we typically have up to 7 or even 8 players involved when we have the ball - one defender depending on what area play is building up on, two strikers one short and one giving longer option, two wider players and two central players looking for quick ball or orchestrating things.
Therefore, we'd effectively be reducing our attacking side from 7 to 5 players, and compensating for it by having an extra CB is not worth it imo. We'd still be suspect at defensive set pieces and just as likely to get caught out by a good pass - playing De Jong and Barry probably squeezes the spaces for the opposition much more than having an extra CB would do, with the added bonus of putting pressure on the player in possession, which a third CB woudl probably not do any better.
