I have a relationship field that will show custom posts from two custom post categories, That works fine.
I want to exclude any of these that are also a third custom post category.
I have tried many things.
this is the latest, It still shows on the left side the post that is marked with the third custom post category
If I understand you want to include posts that are in 2 categories but exclude them if they are also in a third category.
I.E. Show if in category 76 but not if it is also in category 78.
I think that in order to do this you need to use a tax_query. I could be wrong but I do not think the ‘cat’ argument can be used the way you are trying to use it and I also thing that using ‘category__in’ combined with ‘category__not_in’ will work the same way. Like I said, I could be wrong but I think these arguments work as ‘OR’ (meaning category 76 OR not category 78).