Charging Multiple 3S Lithium Ion Packs Together
The first thing I'll warn, is that this is going to be rather complicated to do safely.

Here's one way you may be able to do it:


Make each "socket" an independent multi-cell balance charger. Ensure that these will not be back-powered if power to the charger is lost while the battery is connected.
Make the sockets also branch off power from each battery through a diode to a common bus. Whatever battery is highest will power the bus until they are similar.
Pull power from this bus to power things in blackout conditions. Never draw more than 1 battery can handle.