I've tried that and it pulls posts for either of the cats as stated in the link you shared "category__in means the post must be in at least ONE of the specified categories (an OR query)." The results should be in all of the specified categories.
I'm trying to retrieve posts that's marked in all 3 state1,state2,andstate3, not just state1 or another combo.
For example the posts are agencies, how to search for an agency that recruits cat=females, FL, Walk-ins. Is it possible to search like this?
I don't want to convert to tags bc I may end up needing the hierarchy and it will be a mess to switch back if so.