I think it means there have been 3 new comments either since your upvote or your last viewing of the post.
this post was submitted on 06 Aug 2023
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Memmy - An iOS client for Lemmy
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New comments since the last time you were in that thread
That’s an awesome feature!
It’s new comments
Thanks!
No problem. Glad I could help!
3 new comments?
I found this confusing (as did the op). Maybe instead of using the plus icon and making it stand out, it could just be 41 (3 new).