How can one enable a three tier mode, where SP provides infrastructure, reseller will get its own Cloud Director instance (branded) to resell to their end customers. The reseller gets its own instance with system admin equivalent rights and manages its own tenants
Unfortunately, this isn't possible today. You can configure potentially custom orgadmin and tenantadmin rights that might met your requirements, but this is something you'll need to test.
I suggest putting in a feature request (Contact Us ) to request this specific feature.
It is technically possible to connect multiple VCD instances to a vCenter instance, but we do not have an official supported stance on this architecture. This is due to the VCD -> vCenter connectivity nature from the cells to vCenter. I highly suggest a dedicated vCenter per Reseller since I presume you'll be dedicating hosts anyway.