Posting the same link to 3 magazines is enough IMO. Your /kbin account can be followed via Mastodon and similar services, and Mastodon interprets crossposting as completely different posts.
They both federate and they both look the same I think.
However, Mastodon search works by tags iirc. And kbin/lemmy users tend to not use tags. So discoverability is low.
The Microblog section is supposed to add the magazine's tags to your posts there, though I don't know if it currently does. I remember reading issues about it not being the case, but those were rather old, so not sure about the current status.
A bit of a miscelaneous request, but I would kind of like a single button "x-post" for articles so that I can quickly cross post to related communities. If it's a thread (like this one, and not a link), it would link directly to the thread and and also show the comments of the thread (basically becomes a pointer to an article)