would the client know which one of those three communities is the “main” and which other two should be filtered away.

Imho, it should not “filter away” any of the entries. But rather merge all in the same “meta-post” (or whichever term is chosen). Show those posts a bit differently by aggregating the authors (with a clickable “…” ellipsis if the list is too long but that allows to show them if wanted), and when viewing the content either aggregate the comment threads or maybe add tabs to alternate between instances.