Great questions 
Do you want members in one to be able to freely participate in the others?
No, it would be so unlikely that a member from one would ever want to join or engage in one of the other communities.
Do you want conversations in one to be able to cross over into the others?
No, there will be no crossover. They are completely separate subject matters with different target members.
Can you share any examples of the different features and functionality you expect each to use?
- A single branded landing/home page with three links to the unique communities
- Translation plugin
- SSO
- Topics
- Private groups
- Ability to easily replicate features in one community in one of the others
- Potentially some development work for member so self-select which of the three communities they want to be a member of
- Ability for the Community Manager to make changes across all three communities or across one community depending on their need
- An Analytical Hub for one community (at first) Creating an Analytical Hub using Categories, Approval Process and Upvoting