Third level domain - howto

1. You appear to have servername.com and servername.foo in the same zone. There should only be one domain per zone.
2. If you just forgot to change the first .com to .foo, then thirdlevel.servername.foo. should just be thirdlevel
3. For the value of thirdlevel, you need a . on the end, or it will append the origin for you (assuming the zone file starts with $ORIGIN servername.foo.).

If you have access to the text of the zone file, instead of the usual annoying HTML form that most registrars provide, you want something like the following:

$ORIGIN servername.foo.
$TTL 14400
@       86400   IN      SOA     YOUR.NS1.GOES.HERE.      your.email.with.at.replaced.by.dot.comcast.net.   (
                                        201009091812 ;Serial Number
                                        86400 ;refresh
                                        7200 ;retry
                                        3600000 ;expire
                                        86400 ;minimum
)
                86400   IN      NS      YOUR.NS1.GOES.HERE.
                86400   IN      NS      YOUR.NS2.GOES.HERE.
                14400   IN      A       123.45.67.89
www             14400   IN      A       123.45.67.89
thirdlevel      14400   IN      CNAME   @

The @ gets replaced by whatever $ORIGIN is set to, so you don't have to keep retyping it. I am curious why you can't just use another A record for thirdlevel.