Fortinet Fortigate Add-on for Splunk version 1.4 and Fortinet FortiGate App for Splunk version 1.4 We also have FortinetAR version 1.0.0
in general, you can use one of 2 ways:
1. syslog server to collect the fortinet logs and install a Forwarder to monitor the syslog directories and
2. Heavy Forwarder listening on UDP to the fortinet firewall and sending the data to indexers
in both cases, you have 1 IP that fortinet will send data to, and from that point, data will be load balanced to the 3 Indexers
there are many many articles in this portal and in community regarding considerations on both options.
use your favorite search engine, try something like: "splunk forwarder vs. syslog"
hope it helps