In the fortiview you can check the Fortiview sources(Top traffic sessions aggregated by source) from where you can get an idea which user is utilizing more bandwidth.
Then you need to send that user traffic through the required interface. If you route with IP, if its a static IP then fine, if it is dynamic then the routing will not be appropriate as it keeps changing. Hence, you can configure it with the user instead.
I believe 192.168.3.101 is taking higher bandwidth for you so as suggested you can create sd-wan rule and in interface preference, you can keep only wan1 as you want traffic from 192.168.3.101 go through wan1 only.
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