You would need to create 3 groups for permissions purposes. Then you would need to create 3 categories for the 3 ‘sub communities’
eg.
Create:
Group 1
Group 2
Group 3
Then create:
Category 1 (for Group 1) - Category security permissions are: ‘Group 1 create/reply/see’
Category 2 (for Group 2) - Category security permissions are: ‘Group 2 create/reply/see’
Category 3 (for Group 3) - Category security permissions are: ‘Group 3 create/reply/see’