Something you might consider is to deploy/upgrade a vCenter at site B and then deploy a new vCSA at site C and link them. The vCSA at site B manages hosts in both sites A and B while C manages only C. Pro here is you keep C hosts locally managed due to latency and nearly 1,000 km in distance; con is you need a second vCenter license/seat.
Totally agree and that is the initial thought / plan of attack, reducing the license from three back to two.
Running with the long term result (Site A and B collapsed into a single geographic location), weighing up the pro's and con's of collapsing into either a one or two vCenter server instances.
Taking into account the number of hosts and VM's at site C and the distance / connectivity, is there a guide to assist with defining deployment of a local or remote vCenter server instance.
Are you doing anything other than managing the ESXi host? vSphere replication, Cross Site vMotion? What is your connectivity between site?
Depending on what you are looking at, I might consider 1 vCenter server at Site A or B and Definitely one at C, and put them both in the same SSO so you can do Linked Mode to manage them both from a single interface. That will also open you up to being able to use vSphere replication.
Used for management of the hosts, the primary features being used are DRS and HA.
After digging around a little, the release of 6.5U2 introduces Enhanced Linked Mode with embedded PSC, this looks like the way to go.
As site A will be collapsed into site B, install two instances - one at site A and one at site C
Absolutely look at embedded PSC, external ones are a pain!