Perhaps he was after a triple split load wth three RCDs, three neutral bars and three live bars?

I.e move three MCBs onto its own 3rd RCD but has only spare space for a single width unit like a RCBO but not a double width unit like a RCD.

But yes you do make a good point as the three existing MCBs could be replaced with RCBOs and wired into the incoming live and incoming neutral.