Best way to blog on a shared multisite?

Hi there!
I'm looking for a simple way to have one shared blog across an entire multisite network (3 sites).

The blog would allow 3 different users to post to the blog from the 3 sites (main site and 2 subsites).

I'm new to wordpress, blogging in general so i'm not familiar with how things work. The blog would also need to have different categories that I am able to have different templates for (depending on the category).

Thank you all for all of your efforts here in the forums!

Much Love
Ciro