This page from Apple covers what you need to know… but the idea is this: Daniel Steinhardt posted a video with steps to update the firmware in a G3, and suggested turning on Airplane mode, and restoring bluetooth connectivity in order to prevent calls or such from interrupting the updating process.
Of course, that means that you have to remember to turn that off when you’re done, and I suspect in practice that could be easily forgotten.
The page above tells you how to create a focus (or modify one of the presets). If you don’t allow anyone to call, and not allow apps to post notifications etc. I believe you’ll get the same effect, but it will turn on when you run the G3 app, and turn off when you close the G3 app. Nice! The screen shot below shows you the focus I created for this which called (duh) “G3”. You can see that no people are allowed and that the G3 app is selected so that it will automatically turn on when you run the app. Other details are available on Apple’s page above, but it’s all pretty straight forward. Enjoy!