Hey Andrew and Christian,
Thanks for the ping, and also for the friendly email 
I’m not that active here these days, I’d like to be, but I’m not finding much time at the moment, unfortunately.
Regarding your questions: If you’re interested in multiple outcomes, maybe reading about a multinomial regression would be helpful. I had a look and it looks like @cluhmann previously (How to fit multivariate multinomial model) referred to a notebook: https://nbviewer.org/github/cluhmann/DBDA-python/blob/master/Notebooks/Chapter%2022.ipynb . That seems to link to the Kruschke book. So that’s where I would look. Try to code up a multinomial logistic regression, then see if you can link things back to your problem. I was looking but in tennis I’ve usually just done binary outcomes, not multivariate, so I unfortunately don’t have any code that’s immediately applicable.
Looking at the error you’re getting, it seems like a shape issue. I think your first input should indeed have shape[1]=3 – one column for each outcome – but fight_outcome should probably just be a vector of the length of your data points. Basically, if you have N data points and M outcomes, I think p should be of shape N x M, and observed should be N (please correct me if I’m wrong @cluhmann ).
Hope that at least gets you started, let me know!