Have you reset the newly acquired items before trying to add them to your system? Don’t bother with the Bridge. Then once they’re all on your network you can use the Group feature to play sync’d music to all three speakers. 

No I haven’t. How do I do that? Thank you 

https://support.sonos.com/en-gb/article/reset-your-sonos-product

I’ve got the 3 speakers set up now thanks. The two play 3 speakers are playing. But the play one is not playing music from it although the other two are. 

Are the Play:3s in a stereo pair or are they showing in separate rooms in the app?

Are you able to select the Play:1 and play music to it?

To group speakers/rooms together, click on the symbol to the right of the volume bar on the Now Playing screen.

https://support.sonos.com/en-gb/article/group-and-ungroup-rooms

Thank you all so much. All done

To help others: what was the fix?

All I had to do was reset the speakers I had bought second hand …. Then the app recognises the “new” speakers …. Pair all new speakers …. Check setup and add all the speakers as a group to play altogether