Not a supported configuration for Sonos, the 2 add-on speakers are for surround use, not front.
The Arc is your front speakers for LCR and Atmos upfiring.
I know what you mean about the B&W, my AVR has some weird and wonderful speaker setups in addition to front wide and height/presence/ceiling options. It even has an option for virtual cinema which lets you put all the speakers at the front of the room instead of the usual front/side/rear
Can I use a pair of eras 300 as surround? for the back ?
Can I use a pair of eras 300 as surround? for the back ?
Yes, that is a supported configuration.