You don't need static routes on the FGT when it's directly connected to each subnet, (tagged) VLANs or plain. A router only requires routes to remote subnets that it can't see itself.
Clients in each subnet either need the FGT as default gateway or a route to the other subnet(s). Each client needs to be able to ping its gateway(s) - make sure you've enabled PING on the FGT interface at least temporarily.
As a firewall, the FGT defaults to deny any traffic unless it is permitted by a firewall rule. Your rules look like they should work but if they don't you need to provide the configuration. Make sure the rules are not behind/below other rules denying the traffic, the FGT applies the first/topmost rule that matches the traffic.
Alternatively, you check the Forward Traffic log to see if the rules match and what's going on.