You can essentially set them all to the same security level and use the system command which allows traffic to traverse interfaces with the same security levels. Just go into your int config, give them all a nameif, and then set their security levels to something benign.
There are two variations of the command //same-security-traffic permit inter-interface and <intra-interface>
...
Intra - between the same interface and itself
Inter - between two different interfaces.
The first of which allows a sort of hair pinning to occur,
You'll still need ACLs, to meter which traffic you want to egress each interface toward the other subnets.