Interesting stuff.
Some of this makes me wonder whether it may be better off keeping these as three separate sites. Has that been ruled out? If so, why?
Here’s my thinking – so far, it seems like the motivation for putting these all under one site is for the convenience of the team managing them. But maybe there are other ways to approach that. Applying configuration changes to multiple communities feels like a technical problem that could be solved in other ways, and keeping them separate allows for greater freedom to do things differently in each, if it’s determined that each community as different needs (which could further diverge as the communities evolve).
On the other hand, I see a hint of a few exceptions to how I’ve characterized things:
A single branded landing/home page with three links to the unique communities
Why is this important?
If people in these communities are completely separate from each other, who is it that benefits from seeing these all in one place?
Perhaps the answer here leads to some key insight I’m missing so far.
Again, I feel like this could be addressed in another way. If it’s just a home page with three links, maybe with some content pulled in dynamically, that could be implemented outside of the site.
Now all that said, if revisiting the above doesn’t feel helpful and the decision has been made that all of these should live under one site, we can bring the conversation back to your original questions about how to do that, just keeping what you’ve shared so far in mind as context.