Re: Deploying fortigate 100E with layer 3 switch for vlan
not that much information, how much are you able / allowed to change, why the layer 3 switch if you have a FortiGate that can terminate the VLANs and be the router, ...
if there are no special requirements then i would use the FortiGate as firewall / router, connect via a link aggregate (lag) to the switch for the extra switchports (if you need more then the ones on the 100E).
if the layer 3 switch must be the core, then you just add an extra network between it and the FortiGate and set the default route towards the FortiGate. on the FortiGate you configure routes towards the core switch.