_netblocks3.google.com has no CNAME records.
Question and response
Question
dig _netblocks3.google.com. NS
Answer
The response has no answer records.
Authority
google.com. 51 SOA ns1.google.com. dns-admin.google.com. 873609184 900 900 1800 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0