NHLD is used to distribute traffic between multiple ISP links accordingly. In that perspective, having NHLD device behind your perimeter firewall would require you to isolate those 3 ISPs virtually inside your firewall and create three different L3 domains between your perimeter firewall and NHLD device so that it can load balance traffic between 3 different end points.
Normally NHLD is deployed at the edge where it will allow A10 device to balance traffic between multiple service providers links. In your case you are having another L3 device in the middle which makes NHLD device basically useless since all traffic flow is going to same device regardless.
So to be able to make this approach working, if your firewall supports vdoms or vrfs regardless, terminate each ISP on different vdom / vrf on your firewall and make isolated L3 connection between NHLD device & your firewall. This way would allow NHLD device to assume that there are three different gateways and balance traffic accordingly. Basically perform its task as it supposed to be 
As commented above, for further discussion if you could share a sanitized network diagram, it could be helpful to discuss further.