I think you ask good questions. Namely, how is the best way to present lengthy stories containing multiple fights? It’s a challenge some of us have gone through. I’d say you are right to divide up your story, as 30,000 words is considered a novella. I think 3 sections of 10,000 words is fine. It sounds like your backstories, build-up and continuity parts are important for your story, so you don’t need to give them up. However, you might want to consider subdivisions within your three posts.
Most readers (including myself) have short attention spans. If I’ve learned anything about writing, it’s that the presentation and layout of the work is critically important in attracting and retaining readers. I cannot read a story that is a wall of text with no breaks. In my opinion, the key to preventing readers from leaving is to keep the narrative flowing as quickly as possible, build tension and suspense, avoid dull sections, and know what can be edited out, and keep your text easy on the eyes.
Regarding that last point, when I write a lengthy story, I’ll often include section breaks. I believe this has a psychological benefit to the readers by giving them the impression the story is not as long as it actually is, plus it gives the readers the message that a change is up ahead.
In my story, “Waiting at the Door” (11,100 words), I broke the story into multiple sections headed by Roman numerals. This gives the writer the advantage of jumping ahead in time or changing locations without the need for awkward continuity in your text.
https://www.freecatfights.com/forums/index.php?topic=102332.0My story, “The Poacher 4 (13,375 words) was broken into sections whenever the scenes changed locations like a movie.
https://www.freecatfights.com/forums/index.php?topic=89782.0Six fights in one post seems a lot and might give readers “fight fatigue.” Here’s where subsections might help if you can find suitable points in your story to place the breaks Bonus points if you can find a cliffhanger for your break. Do you need to describe all 12 fights in detail? Are all the fights important in driving the plot? Are all the characters in the fights relevant to the story. If not, perhaps some fights can be short (e.g. first round knockout or quick submission). Can the narrative of some fights be truncated with a general summary, highlights and ending?
You’re a fine writer and I won’t be offended if you don’t go with any of these ideas. You will find a way that works for you. Best of luck!