How to PERMANENTLY group Sonos 100 and 300?

Heya,

as stated in the title im looking for a solution to link 2 sonos 100 and 1 sonos 300 speakers.

i bought them for my father, the first 2 sonos 100 were easy to set up.

i linked them to a room without a problem, they are now listed as stereo pair. the problem is the 300 speaker. as i try to assign it to the same room it just creates a copy of it with only the 300 speaker.

so now i have “room” with 2 x sonos 100
and “room 1” with 1x sonos 300

ideally i would like to assign both to the same room but it just does not seem possible on the iphone sonos configuration app.

i know i can select both rooms in the sonos app to play something but this means i have to do it everytime media changes ( like media from a connected phone, tv or by selecting something in the sonos app)

there is no shot i can explain this to my father so im looking for a foolproof method to couple the speakers. im open to using different apps as the “sonos experience” is giving me low key cancer.

thank you all in advance.