No the external IP is meant or let's say the IP the browser will connect to.
So different ssl hosts are only possible if you can provide a different ip and port combination per cert, e.g.
18.104.22.168 Port 443 -> CERT1 / Host 1
22.214.171.124 Port "444" -> CERT 2 / Host 2
126.96.36.199 Port 443 -> CERT 3 / Host 3.
So in case you have your ispconfig behind a router you need to have as many official IPs as you need + forwarding / NATing them to the appropriate internal address.