Upgrading from flash to moto3

I want to upgrade flash website to moto 3. I want to keep flash site active for 3 months till I finish work on moto3 template.

1. Do I then install moto 3 template in a sub-folder of main site domain?

2. If so, what domain address - with or without sub-folder - should I use at the time of activation of moto 3 template? Will that need to be changed later once I move my files to public html?

3. After I finish work on new moto 3 template, do I just move all files from subfolder to public html?

Thank you for your help and support.

Best Regards,