Unfortunately, the answer to that question is no. The Sonos Era 300 can only connect to Sonos tech from Gen 2 or later, such as the Arc or the Beam. This is because the Era 300’s multichannel rear speaker design is built to support Dolby Atmos, something you can only get in Gen 2 onward. The Sonos Playbar and Playbase, on the other hand, are Gen 1 tech, which means they don’t have the full tech specs necessary to support the Era 300. If you try to connect them, it simply isn’t going to happen.
Incidentally, this support hiccup also applies to the lower-end Sonos Era 100 speaker. While the Era 100 is not equipped with Dolby Atmos, it’s still too new to properly interface with either the Playbar or Playbase. However, the Era 100 can connect with any generation of the Beam speaker, even if it’s Gen 1, alongside the Sonos Ray and Arc. If you’ve got an older Beam speaker, you may want to pick up a couple of Era 100s in lieu of the Era 300 to ensure full and proper connectivity.
As for the Playbar and Playbase, there is unfortunately no way to facilitate a connection between them and an Era 300 or 100. In fairness, the original Playbar has been around for a decade now, with the Playbase only a few years behind, so it was only a matter of time before the two reached their sunset years.