iRule to forward traffic based in state pool member

So you have a gateway pool, with gateway_icmp monitors (maybe with a transparent monitor).

You need to select the appropriate SNAT IP address based on the selected outgoing router.

I would apply priority group settings so that you have a preferred router (10.0.0.1) and fallback routers (10.0.0.2, 10.0.0.3).

    when CLIENT_ACCEPTED {
      if {[LB::status pool default_gateway_pool member 10.0.0.1 0] eq "up"}
         Preferred router 10.0.0.1 up, so use correct SNAT
        { snat 10.1.0.1 }
      elseif {[LB::status pool default_gateway_pool member 10.0.0.2 0] eq "up"}
         Preferred Router not up, Fallback1 router 10.0.02 up, 
        { snat 10.2.0.1 }
      elseif {[LB::status pool default_gateway_pool member 10.0.0.3 0] eq "up"}
         Preferred router, Fallback1 router not up,
        { snat 10.3.0.1 }
      }

I hope this helps.