Yes, the main benefit as you mention will be the superior stereo separation, but the Era 100s also give you the option to add a voice assistant (Sonos Voice Control and/or Alexa), have Bluetooth and can use the USB-C with additional Line-In adapter, if you wished to connect an external source.

Very happy with my stereo pairs of Era 100s and the sound from them.

Edit: The Era 100 stereo pair can also undergo Quick Tuning if you only have an Android device to tune the speakers to your room.

What about sound quality vs the play 3 ?

It’s been years since I’ve listened to a Play:3 as replaced them with Ones, then the Ones with Era 100s.

I think a stereo pair of Era 100s sound great for music, even without a Sub attached. They fill my medium sized living room as they have a wide soundstage. Most reviews have rated them highly for music so I’d say you won’t be disappointed if you upgrade from the Play:3.