What controller app are you using, and what firmware? You may need to speak with Sonos: if a speaker gets too far behind the current versions, it’s too big a “jump” to update in one step. Play:3’s can still operate though. 

Hi, I bought a new phone recently, before this phone I used the Sonos app no problem but the app doesn’t support play 3

The Sonos Play:3 *is* supported by all Sonos App’s currently.  What phone is it? 

Does the Sonos App icon say S1 on it, or Just Sonos with a black background or Sonos with a gold background? 

If your phone runs the app, it runs the app and controls any and all compatible speakers. Compatibility between controller and speakers is here https://support.sonos.com/en-us/article/sonos-app-version-compatibility

But it has always been necessary to keep controllers and speakers updated in sync. As I already said, for a long-out-of-date device you’ll need help from Sonos support. Did you speak to them yet, and what did they say? 

Had to change my router and trying to reconnect the old Sonos Play 3 to the iOS App.

Eventually managed to reset the speaker although the App still does not find the Play 3 speaker irrespective of all the advice I have seen online. Had to physically connect the Play 3 to the network with a UTP/Ethernet cable just to reset it although even when connected the speaker still does not appear as an option to connect to. 

I tried the earlier version of the Sonos App as that is still installed on my iPhone alongside Sonos S1 release.

Is the speaker finally no longer supported in any of the Apps as that would mean the final component of a significant Sonos investment is destined for the tip

My Play 3s are working just fine.

Hook up the Ethernet again.

Power-cycle the Play 3.

Wait 5 minutes and submit a diagnostic.

Call Sonos Support with the number.