assuming you did the site-2-site IPsec between the FortiGates with the wizzard and the HQ FGT has all the routes to the local networks in place (192.168.11.0/24,192.168.12.0/24,192.168.13.0/24), it should be fairly straight forward.
On the branch FGT you need to configure the routes to the networks on the HQ site going to the IPsec interface:
Via GUI navigate to Network -> Static Routes -> Create New
Repeat that for 192.168.12.0/24 and 192.168.13.0/24
Now create policies to allow the traffic between the networks and interfaces.
For example: From IPsec interface -> port1, and vice versa
On the HQ FGT you need a static route to the 192.168.100.0/24 network, also pointing to the IPsec interface.
Make sure you have policies to allow traffic from the branch (IPsec interface) to the local networks (port1), and also from the branch office to the local networks.