will I be able to access all current channels on my old Roku 3 on a new Roku? Will I have to re-enter all provider credentials, passwords, etc? What if provider limits number of devices?
If you link your new Roku to the same Roku account as your Roku 3 all your channels/apps will be automatically installed. Depending on how you subscribe to premium services you may have to enter your credentials again. Providers that limit the number of devices generally have a way to add/remove devices on their website.