That is called an inverter.
Yes, you can use a 230 volt device on a 240 volt supply.
No, it will destroy the 3 volt device.
No. You will need a transformer to step up the voltage and you will possibly need to convert the plug.
Yes
It depends entirely on the device. Give it a try.
Yes.
No, a 240 volt device runs on 240, and a 120 volt device runs on 120. Attempting to run a device on incompatible voltage results in damage.
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Use a voltage devider or a transformer.
For higher current applications you would need a power transformer, for lower current applications you would need an adapter transformer.
It can be done with 90% efficiency.
No, the receptacle's rating is 240 volt and that is the maximum voltage allow to be applied to that device. To prevent this condition from happening 277 volt receptacles and switches have a larger box that they fit into. The retaining screws are set apart further that a 240 volt device which makes it impossible to install a 240 volt device in a 277 volt junction box.