For more detail see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=871602#10 and https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=855268#26
In short: do not use systemctl or its "service" alias with Squid-3. Either use the init Script provided by the package, or 'squid -k' commands directly.
Amos