Fortigate 100E act as back bone core router and firewall

Hi!

We have two Fortigate 100E active/passive and three switches. As I dont have any core redundant switches so I want to implement redundancey in the FW. 

The SVI lives on Firewall and the ports are trunk between three switches and Firewall. As FW has 16 ports acting as internal switch. I want to use 12 of them for this topology.

Can any expert give basic idea about its implementation?

Thanks