The Arc has front right, center and front left channels.

The 300s have the rear right and left channels.

For Atmos that changes a bit but the speaker positions don’t change.

The Sub placement is a bit different as it is fairly non-directional. You should try multiple places in your room and decide which sounds best to you.

In general, in a corner gives more sound but possibly uneven in different seating spots. Along a flat wall, more even but less sound. You can try multiple positions along the wall looking for a sweet spot. Near an opening to another room, usually less sound, how even it will be you have to try it and see.

I haven’t tried flat but if you have a spot where you can try it and get power to the Sub I’d sure try it.

I have a cloth cover over the Sub’s ports to prevent critters or kids from adding rattly things to the Sub’s innards.

https://www.sonos.com/en-us/guides/sub

Select a location

Choose a location—Sub is designed to provide optimal sound wherever you place it.

• Use the acoustic slot as a handle to lift Sub out of the box. You should always bend your knees when lifting heavy objects to avoid undue stress on your back.
• Make sure at least one side of the slot is free from obstruction. If you place it vertically against a wall, don’t block the front opening.
• If you place Sub horizontally on the floor, don’t put anything on top of it. Leave at least 1" clearance along the top of Sub if placing under furniture.
• Sub is designed to sit on the floor. You can place it in a corner, against the wall, behind, under, or next to any piece of furniture on any kind of floor surface.
• Position Sub either vertically or horizontally. If you’re going to use it horizontally, you can attach permanent felt feet. See Attach feet (optional) for more information.