Each firewall needs a dedicated interface for the HA3 link.
The PA-7000 Series, PA-5400 Series, PA-3400 Series, PA-3200, and
PA-1400 Series firewalls use the HSCI port for HA3. The PA-5200
Series firewalls can use the HSCI port for HA3 or you can configure
aggregate interfaces on the dataplane ports for HA3 for redundancy.
On the remaining platforms, you can configure aggregate interfaces
on dataplane ports as the HA3 link for redundancy.