Hi @DavidEJM
Welcome to the Sonos Community!
Out of sheer curiosity, I took an early look at your existing system (I’m supposed to wait until the conversation has died down) - I’m glad I did, as I had assumed the same as @Airgetlam in that you were still on S1.
Your existing Play:3s are already on S2. It sounds like your “new” Play:3 is probably on S1, but it might just be on an older version on S2.
The main issue is your Bridge - it has not been used since you updated to S2, and it seems very likely the only thing it is doing is getting in your way. Even if you were still on S1 and actually able to use the Bridge, we’d still recommend that you not do so.
Please permanently power-off and disconnect the Bridge. If this makes your whole system go offline (which seems very unlikely, but I’m saying this just in case), please temporarily connect an existing Play:3 to the router instead, wait for the system to come online, then go to Settings » System » Network » Manage Networks » Update Networks and follow the on-screen instructions to get your Sonos system on WiFi. The app will tell you when to disconnect the ethernet cable.
You should then be able to add the “new” Play:3 to your system without issue. If not, I recommend rebooting your router by switching it off for at least 30 seconds.
Importantly, you don’t need to buy anything, and, frankly, never did.
I hope this helps.