Can I use the “Lightning to USB3” to connect a wired mouse and keyboard to my iPad?

I just tested this on my 6th generation iPad, with an Apple USB 'short' keyboard and a generic mouse plugged into that.

I get an alert that the device uses too much power, unless I also keep the iPad plugged into the mains, using the Lightning port on the adaptor.

Your mileage may vary on newer iPads. But you'll likely need the USB3 adaptor that splits USB and Lightning, not the straight-through Lightning to USB2 adaptor.

https://www.apple.com/uk/shop/product/MK0W2ZM/A/lightning-to-usb-3-camera-adapter


Yes, that shouldn't in itself be a problem.