It’s up to you!
All you have to do is connect your 3 TVs, then name them something like Roku 1, Roku 2, etc.
After that, you can control each TV individually. BUT, if you want to control them at once, you would just put your 3 devices into a “group.” This is easily done in the app. Then that would allow you to turn on and off all 3 TVs at once. :)
I have the Toshiba that has Fire TV built in, similar to the Roku version, and it was so easy to set up my remote. You shouldn’t have any problems with what you’re looking to do!