On the holding player I see what you mean, we don't have one as an out and out option. Can Gunnar or O'Keefe be trained to nail this role? Before we signed O'Keefe, I was under the impression he was this type of player anyway. Perhaps were better off going with what we have?
One the essential things that needs to happen if we play three at the back, is those three CBs need to be adaptable. When we don't have the ball, there will very frequently be passages of play where one of those three must step up into midfield to cut out passing channels or put pressure on the ball. I feel this insurance policy behind the deeper midfielders means that the defensive mids in our system are less reliant on defensive acumen. Though it's not ideal if they are not a natural fit for the role they are playing of course.
A big positive for us is we have centre backs who should do well as one of three CB as they are good all round footballers and have the smarts to know when to push up. Like you say Gringo, we comfortably have the right types of CBs already at the club.
I imagine Morrison, because of his height, is likely to end up being the centre defender. I haven't caught up with the game from yesterday. Did he play in there?