Is ot just me or i cant find chapter where black bulls safe the 3 people from the diamond ...

me too

I heard that the story about that is not in the manga but a short novel written by the author. ...

Dude, same. I was so confused lol because there's no way I would have missed that. They're from a light novel released by the author and there's no translation
as of yet official or otherwise. There's actually a second one that came out earlier this month I believe.