Here's a translated post from the German Forum that shows how to connect A and C through B (headquarters):
Location A: 192.168.11.0/24
1 Remote Gateway
Remote Gateway: [Public IP of Location B]
Remote Networks: 192.168.1.0/24 and 192.168.100.0/24
Local Networks: 192.168.11.0/24
Location B: 192.168.1.0/24
2 Remote Gateways und 2 IPSec connections
Remote Gateway: [Public IP Location A]
Remote Networks: 192.168.11.0/24
Local Networks: 192.168.1.0/24 and 192.168.100.0/24Remote Gateway: [Public IP Location C]
Remote Networks: 192.168.100.0/24
Local Networks: 192.168.1.0/24 and 192.168.11.0/24
Location C: 192.168.100.0/24
1 Remote Gateway
Remote Gateway: [Public IP Location B]
Remote Networks: 192.168.1.0/24 and 192.168.11.0/24
Local Networks: 192.168.100.0/24
And, if I wanted to route all traffic through Location B, I simply would make the following changes:
Location A: 192.168.11.0/24
Remote Networks: add 'Internet' to the other networks
Location B: 192.168.1.0/24Local Networks: add 'Internet' to the other networks for both IPsec Connections
Location C: 192.168.100.0/24Remote Networks: add 'Internet' to the other networks