As far as i know this is not possible by default. You would have to do some trickery to get this. The question is, do you want/need the screen type to look like this that badly? 

You could get the data on screen using double references from table A to B. Reference B(top right) which is the normal reference. And Reference B (bottom) which would be a gui reference with a variant of B. Then set C as a detail of reference B (bottom) However the most tricky part is getting C to refresh based on B (top right).

You are probably better off selecting a different screen layout for this. However non-rectangular detail components would be amazing. Currently the screen types are very restricting, having some more advanced options for this or setting up a complete screen type from 1 screen would be a welcome addition.  Not sure if that’s possible but you would have my vote if you posted an idea for that.