Hi.  Sonos “rooms” do not necessarily correspond to physical rooms in your home.  A Sonos room is just a label for a speaker, or for a specified combination of speakers (such as a stereo pair or home theater setup.

To achieve a home theater setup you would start with a soundbar and two surround speakers, let’s say you have called them Arc, Surround Left and Surround Right.  You then just go into the settings for the “Arc” and choose to Add Surrounds.  Follow the prompts and you will end up with a single “room” visible in the app - namely “Arc”.

Of course, you could start out by calling your Arc “Living Room” or whatever else works best for you.

It is not clear to me how the 100s and 300s relate to each other in your setup.  But you can only have 100s OR 300s as your surrounds, not 100s and 300s.