I would go out on a limb and posit that it's almost entirely psychological. At this point, I'd guess that Sá, Kilman, and Dawson as that "center triad" (there's probably an actual tactical word for the GK + CBs that I don't know) don't have a lot of faith in each other making the proper decisions; ergo, it takes a lot less pressure to make them feel exposed.
Adding the 3rd CB, especially an athlete like Toti who can get around if needed, likely raises the confidence floor for everyone, as there's less fear that a single error will lead directly to a goal.
All speculation, but anxiety is a powerful thing, innit.