L2 links redundancy b/w two sites

Dear Concern,

We have two offices located in different regions, we have three L2 links for connectivity between them currently we are using 1 link on runtime configured 2nd for backup and 3rd is administratively down. If both 1 & 2 goes down at time we switch traffic to third one manually. All these links are terminated on our distribution switch. In this way are wasting our bandwidth of two links which we have precured form ISP's.

Both sides we have foritage firewall as edge devices please suggest the feasible solution on fortigate firewall for using all three links simultaneously with redundancy, so that we can eliminate network loops and human intervention for switching. 

Find below diagram for your reference. Your valuable feedback is highly appreciated.