You say you already have 3 Rokus? Are you using them? If not, then: yes: a Roku in a TV works as a Roku. As for Apple TVs, the only things I know of by that name are players (like Roku players) not entire TV sets. Therefore, they would just be plugged into different inputs on the TV and wouldn’t affect each other. (Just like having a blu-ray player and cable box on the same TV – you just switch inputs.)
I would suggest using the Rokus and getting comfortable before you decide to cancel anything. Give it a week or so to see how you like them, since you have them.
Hmm, I reread your message and now I am confused that (only) ONE of the Roku TVs has Roku OS built-in. I don’t know how to interpret that.
Keep in mind that if you use any channel apps that you have logged into with your cable credentials (ie: those apps marked in the channel store as: Cable or Satellite subscription required) will soon cease to function after you cancel cable. Everything else won’t care about your cable TV status.
