Please try:::::

 given::
 echo $ADMIN_IPS 
8.8.10.1,8.8.10.2,8.8.10.3

and Input file /tmp/myvar given with contents as below:::: 

-A INPUT -i @PUBLIC_INTERFACE@ -p tcp -m tcp --dport 80 -d @PUBLIC_IP@ --syn -j ACCEPT

-A INPUT -i @PUBLIC_INTERFACE@ -p tcp -m tcp --dport 22 -d @PUBLIC_IP@ -s @ADMIN_IPS@ --syn -j ACCEPT

-A INPUT -i @PUBLIC_INTERFACE@ -p tcp -m tcp --dport 443 -d @PUBLIC_IP@ --syn -j ACCEPT
 
 
 while read j; do savedline=$(echo "$j"|sed 's/@PUBLIC_INTERFACE@/eth0/'|sed 's/@PUBLIC_IP@/1.1.1.1/'); for i in $(echo $ADMIN_IPS |tr ',' '\n'); do echo $savedline|sed "s/@ADMIN_IPS@/$i/";done|uniq;done < /tmp/myvar

 Try and let us know.