Make sure you stream the music on the AirPlay-compatible Sonos device first before grouping the Play:3.

thank you for answering!

actually when I select the devices from airplay icon i only can see the airplay compatible devices so the play3 cannot be chosen. So far so good as I know it, but how can I make it sound all over the system including the play3? From the sonos app menu I have correctly set the consolidate function

Use the Sonos app to Group the rooms you want included. 

Thanks for the idea, I already created group on sonos app but they do not show up as you can see on the print screen I sent 

Are you using the System icon in the Sonos app to bring up the rooms/groups? I don’t recognise your screenshot as a Sonos page. 

More on groups here: https://support.sonos.com/en-us/article/group-and-ungroup-rooms

and Airplay: https://support.sonos.com/en-gb/article/stream-airplay-audio-to-sonos

The ‘group’ won’t show up on the AirPlay 2 screen, just the one Sonos device that is AirPlay 2 capable. It will then receive an AirPlay 2 signal, and as it is grouped with the other PLAY:3s , the signal will play there as well.
 

I’m currently doing this with a stereo signal sent by AirPlay 2 from my AppleTV to a Sonos One, and I’ve grouped a stereo pair of PLAY:3s with that Sonos One, and I get sound from all three speakers. 

And AirPlay here: https://support.sonos.com/en-gb/article/stream-airplay-audio-to-sonos

The screen you saw is the one on my phone when I click the AirPlay icon from YouTube 

This is my problem! I cannot see the group. On top of this, as already written, despite being grouped on Sonos app, the other play3s do not play any sound when I reproduce music from YouTube in my iPhone. Please help!

The group isn’t visible for AirPlay. AirPlay only sees AirPlay capable speakers.

Can you try to play to one of your AirPlay enabled speakers and then group that with the Play 3’s. Do you still not get any sound?

It might be me, but you don’t seem to be giving clear details.
 

What are you doing in the Sonos app and what are you doing in your player? 

In your player, start the airplay stream to your Sonos speaker. Then switch to the Sonos app. Now try grouping the speaker already playing and your other speakers/group, with that airplay stream as your source. 
 

If you do the above sequence of steps, what happens at each step?