From all the responses above, the debate seems to be revolving around CB as a position in the formation vs CB as what the player has built a reputation as / has qualities of.
Acnumber9 seems to be referring to CB as the position in the formation, while Stepic, Drainy etc are referring to players who have built a reputation / mainly played as fullbacks previously but are now playing in the CB position in the formation.
In the past, we had Roy Keane playing as CB for us. Obviously Keano is most known for being a CM, and obviously has more CM qualities than a CB, but in that specific formation, he was playing as part of a 2.
My view is, regarding formation, our system is pretty fluid in the sense that looking at the positions that our CBs take up, we have 1 CB consistently pushing up higher than the other 2, such that he is functioning essentially as a CM. So at that point, he is playing as a CM rather than a CB. Then when our formation morphs, e.g. when defending as a 5-4-1, then we do indeed have 3 CBs. So I would be hesitant to claim that we are playing with 3 CBs all the time. Previously with a more rigid 4-4-2 structure, I think we rarely saw Ferdinand or Vidic venture past the halfway line, whereas having Yoro or Shaw do so seems to be a staple of Amorim's formation.
The key question is, and I think this is what most posters issue is, is whether the players playing as a CB have the qualities to be a CM, e.g. like if Keano were to play in the formation as one of the 3 CBs, but push up as a CM, we would be confident that he would be up to the task. It is questionable whether our line-up of CBs have the qualities to be a CM when pushing up. I think currently, only Licha has the right mix of vision, ball-playing and defensive nous to do so. Now that he's back, I hope to see more situations like when he scored the goal against Liverpool last year! *fingers crossed*