Is Play 3 "old" technology?

There is no reason not to is you do not care about google or alexa or airplay2. The pair or play 3 sounds better as a pair then a pair of sonos ones imho. Especially if you place them properly by a call. And I hate that if you decide to crank the ones, the annoying distortion prevention keeps turn down and up the volume. I have play3 paired and sonos one paired. And the play3 stays in the living room as they just do a better job over my sonos ones that live in the bedroom.

My other idea is to buy another Sonos One and use those for surrounds and move the 2 Play 3's to another location to stereo pair.

I would go ahead and buy a sonos one directly from Sonos so that you get the return policy. Try out the stereo pair, and using them as surrounds in the rooms you would use them in. If it works well enough, keep the Sonos One. If not, return it and get a Play:3, since you already know how that's going to work.

As far as technology goes, having a Sonos One in your system pretty much makes your system available to the latest tech. Alexa and Google integration won't need that, but airplay 2 will. No one can really say for whatever upgrades may come in the future. That said though, if use the Sonos One, you will lose it's ability to pick up an airplay 2 signal. The playbase will work with airplay 2, but the playbar will not.

Good idea. I have a Playbase so it sounds like I'm set for Airplay 2.

Speaking of "old" tech, does Sonos still update the firmware on the Play:3?

Of course, yes.