Old man, it’s hard to accept your theory that a community loyal to halachah would make a change, even “a ‘minor’ change mid’rabanan to cut from six to three”, based only on “sociological neccessity”. There would have to be a m’kor/sevara, although the sociological necessity could definitely play a role.
It’s entirely possible that there were those who defined “seuda acheres” as the shortest seuda interval in a given time/locale (similar to Mizmor L’Dovid), and even explained at least some Rishonim’s six hours in that context.
It should be noted that the Chayei Adam (127:10 IIRC; I’ll try to find it and link it later if I have time) notes a legitimate minhag to wait “eizeh sha’os” (which would include three), and says it is legitimate to be meikil in case of necessity. He does not talk about “sociological necessity”; that expansion would be your chiddush.