Multi-site VPN, site A (server side), B (client), and C (client).
Also have software clients added to the OpenVPN server side A.
A, B, and C, and talk to each other because you are exporting A, B, C's networks under exported hosts/networks.
The software clients can only talk to A, however you want them to communicate to all sites.
If you want the software clients to also talk to B and C, you will need to add the address pool of the software clients to the exported hosts/networks.