First, make sure you have added the Play:3’s as surround speakers:
https://support.sonos.com/en-us/article/add-surround-speakers-to-a-sonos-home-theater
Next, check to see that you are getting a 5.1 signal. You can see that on the lower left of the Now Playing screen for the Arc room. If it doesn’t say something like “Dolby Digital 5.1” or “Dolby Atmos” then you aren’t getting 5.1. How to fix this? It’s either the source, or the TV isn’t passing it correctly. First, make sure the source you are using is actually DD 5.1 (NOTE: YouTube videos are almost always PCM 2.0. Regardless of what is listed, they are always downgraded to stereo).
If the source is actually 5.1, then it’s the TV. List the year, make and model number and we can get the best settings for your TV.