If you try to install squid using "Click here to have it downloaded and installed using APT" in Debian 8, webmin tries to install package "squid" instead "squid3". Then installation fails.
It can be fixed adding to /usr/share/webmin/software/apt-lib.pl in update_system_resolve function this lines:
$name eq "squid" && $gconfig{'os_version'} >= 8 ?
"squid3" :
Thanks - this will be fixed in the next webmin release.