I assume you mean you wish to bond them all together into a Sonos Home Theatre 7.1.4 surround sound setup. Post device setup/registration, just go into the Arc room settings in ‘Settings/System’ in the mobile Sonos controller (S2) App and add the Era 300s as L/R surrounds and then add the Sub.

Note the Era 300s need to be unpaired and standalone separate ‘Sonos rooms’ in the App before adding them to the Arc room. So if you have them stereo paired then they must be separated before you bond the devices together.