Good day.

I have 3 axpert inverters parallel in single phase and a 3 phase 40kva generator. Can my electritian connect 1 phase per inverter for backup power to charge the batteries and provide single phase through the inverters?

Thanks 

No, that's not going to work. The inverter wants to close its transfer switch and connect the input to the output. The outputs are tied together in a single phase system, but the inputs are 120 degrees apart with 230VAC between them. In all likelihood the inverter will refuse to use the AC input, but it could also blow things up if you try this.

You'd have to either reconfigure the inverters into a three-phase system, or get a separate three-phase charger.