Can the acting PIC sit in the back?
Can the acting PIC sit in the rear seat?
There's no regulation or FAA interpretation I can find that requires the acting PIC to sit in a seat with access to the controls. For flight instruction specifically, 14 CFR 91.109(a) requires dual controls (usually) and the instructor must be at a &quot;pilot station&quot; (14 CFR 61.195(g)(2) and Williams interpretation, 2018). So no instructing from the rear seat.
Of course there's also 91.13 (careless and reckless operation). That's a deliberately broad regulation and I have no idea how risky your operation might be considered. If A and B already have experience in the aircraft and they'd be fine to fly it without C anyway (from the FAA's point of view), then arguably there's nothing careless or reckless happening. On the other hand, if something goes wrong and lawyers get involved to determine liability then it's anyone's guess what could happen.
Can all three pilots log PIC time?

A: yes, as sole manipulator of the controls (61.51(e)(1)(i))
B: yes, if they're giving instruction (61.51(e)(4)). That means A and B need to log it as instruction time in their respective logbooks (61.51(h)); they can't just say &quot;let's call this instruction so we can both log PIC&quot;.
C: no, acting as PIC doesn't automatically allow you to log PIC time. 61.51(e)(1)(iii) doesn't seem to apply here because in your scenario neither the aircraft type nor the operation require a second pilot