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.
Yes... I honestly wish using and subscribing to tags was a more of a thing, since those are cross-instance, and multiple communities with overlapping topics could just make use of overlapping tags. And they also have the advantage that you can add multiple tags to the same post, no need to repeat the post.
Right now you can visit https://kbin.social/tag/technology/threads and see all threads tagged as technology, cross instance and cross magazine, but you cant subscribe to that (but it should be possible to implement at some point though, Mastodon does it).
There's also c) have people start using tags like #technology more frequently in their technology related posts, and implement support for subscribing to a tag (that's something Mastodon already has, so it should be possible).
Tag thread feeds (like https://kbin.social/tag/technology/threads ) show content cross-instance and cross-magazine, and they also have the advantage that you can add multiple tags to the same post, no need to repeat the post to have it appear in multiple tag feeds.
do you say it as kay-bin or k'bin? (lemmy.world)
m.youtube.com/watch?v=SXUDs0ee_w0
Is there a way to deal with crap like this yet?