How to use irule to control 1 IP subnet by only utilizing 1 ISP link.

I assume you already have one pool on the LC that contains all 3 routers for ISP3. I'd create another one with a name like "pool_isp3" and add its router as well as "pool_isp2" which contains its router.

Then, on your IP_Forwarding VS to which you have the default pool that contains all 3 routers, configure and attach an iRule like this:

when CLIENT_ACCEPTED {
if { [IP::addr [IP::client_addr]] eq 192.168.3.0/24 } {
    if { [active_members pool_isp3] > 0}  {
         pool pool_isp3 }
     else { 
         pool pool_isp2 }
     }
}

Something like this should work...what's your plan if both ISP2 and ISP3 are down? You might have to add another else in there. I'll let others chime in on how to optimize this rule üòõ