Lemmy is more of a Reddit alternative, where you have the focus on communities/categories (similar to "subreddits") to which you can subscribe to, while Mastodon is more a Twitter alternative, focusing on users to follow.
Kbin is actually slightly special.
It is also running on the Fediverse network, but can communicate with multiple other services. Meaning, it can show you content from all Lemmy instances (called Threads) and content from Mastodon (called Microblog). You can see this thread there as well.
By using Kbin, you have access to both worlds, Lemmy and Mastodon. In addition, it supports even more of the so-called "ActivityPub" services, like Pleroma or Peertube.
While I love the implementation of Kbin, it still has small hiccups here and there - it feels less reliable/stable. Some content isn't fully updated, like this post missing most of the comments at the time I posted this. They even warn you about it: "The magazine from the federated server may be incomplete. Browse more on the original instance."
I had the best experience so far by using a Lemmy instance for Lemmy and a Mastodon instance for Mastodon. Waiting for Kbin to be more mature.