I would add a Sonos Sub first. Then add a pair of Sonos One SLs as surrounds. If you have a larger room, you can add two Subs as long as one Sub is a Gen 3. Adding a Sub and One SLs as surround speakers will greatly improve your audio experience for both watching TV/movies and listening to music.

TIP: If you add surround speakers to the Arc, be sure to set the Music Playback setting under Surround Audio to Full.

For mostly TV use the One SLs are great as surrounds but if you listen to a lot of music in “Full” instead of “Ambient” mode you might be pleased with a pair of Fives in the rear. Far too little gain in TV mode or Ambient to be worth it though.

The Sub is a good choice too, one will do it for a lot of rooms and you can easily add a second Sub as long as one of them is Gen 3.