Is it possible to combine a 3 kW and 5 kW inverter to achieve an 8-kW inverter array or does the 3 kW only support 24V DC inputs, thus making them “unstackable”?

If you plan to wire more than one unit in parallel, they all need to be the same model number. Up to 9 units may be wired in parallel on a single phase and they must ALL be connected to the same battery bank if a battery bank is used.