Sonos Era 100 alexa group doesn't work (3 piz of era 100)

Hi everyone, I've searched through various topics but I don't think I've found anything. Basically, I first bought two Era 100s and put them in the same room, and with Alexa everything worked perfectly, including the thing I'm interested in: raising and lowering the volume globally with Alexa. The same goes for the physical slider on the speakers... basically, everything perfect. Then I added the third Era 100, but this time I created two rooms: the first with the two Era 100s in stereo, and the second with the single Era 100. Then I grouped them into a group... from now on, after countless attempts, with Alexa I CANNOT make the group volume increase or decrease in unison. Do you have any idea how to fix it? Thanks.