I wouldn’t worry too much about them “aging out” especially the Play:5 (Gen 2). There is a reason why a used Play:5 still sells for around $400. It’s still the best Sonos speaker (along with the Five) for playing music. I have a Play:5 and I expect to use it for many years to come.

Although I have difficulty determine which has a better sound between my PLAY:3 s and the Sonos One.

There’s also the consideration of which OS you’re running. At some point, S1 will be at a more serious disadvantage than it is currently, as it will not get feature updates. . On the other hand, both my PLAY:1s and PLAY:3s operate under S2 without issue.

As @GuitarSuperstar suggests, the PLAY:5 Gen 2 does also operate under S2, as it has enough RAM and CPU to do so.,

Hi.  There is no speaker called a Play One.  Is it a Play:1 or Sonos One you have?  Are you running Sonos S1 or S2?  Do you know if your Connect:Amp is gen 1 or gen 2?  (If you are running it in Sonos S2 then it must be gen 2).

I was disappointed with our Play 3, not an improvement over our Play:1's or Sonos One imho.

Having Play 1s, Play 3s and Sonos One SLs I’d go for the SLs for any further small speaker needs, the Fives for bigger spaces.

The SL sounds good and looks a lot cleaner than the Play 3.

Used I’d not rule out a Play 1 but I’d really lean to the newer SL option if affordable.

That may be because the Play:3 is the earliest of the speakers you list.  

Referring to the bold part above, are all these speakers in the same room?  Adding more speakers in a room is not necessarily going to be an improvement, particularly considering trueplay tuning involved.  If the ‘one larger speaker’ is for a different room, then your existing speakers, then ok.  If you want to add more speakers to the kitchen, then I would not do that.  The best way to improve the sound would be to add a sub bonded to the two play ones, I would guess, but hard to say without more information about the space.