I finally after THREE HOURS got a used Play3 downgraded from S2 to S1 and successfully adopted.

Initially using the S2 app (now just called SONOS app) I went to Settings Gear - Visit Help Center - Legacy S1 App - Downgrade Product to S1 and it said it successfully downgraded the speaker, but it didn’t really.  I could never adopt it using the S1 app. I'd go back to the S2 app and it would find the Play3 eligible to downgrade again.

I rebooted my Samsung Galaxy S25 phone, factory reset the Play3 many times, tried setting up a 2.4G WiFi network (as some said that helped) and even connected the speaker with Ethernet, all to no avail. I was about to give up and return the speaker, but finally got it working with the following tricks:

I left the Play3 plugged in with Ethernet. Using the S2 app I adopted the Play3 to a "New" Sonos system config, separate than my existing S1 devices. It found the Play3 and adopted and then I UPDATED THE S2 FIRMWARE on the speaker. It failed the first attempt, and I had to readopt and and then push updates to it. THEN I used the S2 app to downgrade it, which then connects your phone directly to the built in WiFi on the speaker and pushes the downgrade firmware. This also failed, and I had to do it a few times. It eventually connected and showed a status bar, pushing the old firmware. I was then able to FINALLY adopt it to my S1 app and it now works. Sheesh!!! Hope this helps someone down the line...

S