Kindly let us know the deployment mode of the link balancer.
If it is firewall disabled mode, you can assign an IP address on the WAN3 interface and ASA on the same public IP address subnet. ( it has to be different on ASA and WAN3 interface).
The gateway of the ASA should be the ISP router's IP address ( this would be the gateway of the link balancer's WAN3 interface as well).
Link balancer's LAN interface IP address is insignifcant on firewall disabled mode. It is just used for management access alone.
Please add all the available IP address on the Public subnet (excluding WAN IP and gateway IP address) under additional IP address section on WAN3 configuration.
Refer to the below link as a reference to deploy Link balancer in firewall disabled mode.
Do let us know if you have any questions.
KaushikGo to the full post