Edit: I am releasing Boost for Lemmy, you can pre-register to get notified when it is available. In the meantime you can create an account and join lemmy.world/c/boostforlemmy
This is where I get confused with the federation part. If I’m signed up on kbin and masterdon do I need to sign up on lemmy as well or is it pointless having 3 signups if you are able to see all posts?
It is useful to have just 2 logins, one for kbin (or lemmy) and one for Mastodon (or Calckey).
You can see content from Mastodon on kbin but it's not the same experience since there's no timeline, hashtag follow, etc. Similarly you can see threads from kbin in Mastodon but the UI is not very nice.
For kbin and lemmy, different logins are not needed since they share the way they show information. So you can seamlessly browse lemmy content in kbin without worrying where it was originally posted.