Apex domains cannot have CNAME records.
Question and response
Question
dig 3bb.co.th. NS
Answer
3bb.co.th. 5 NS ns1.3bb.co.th.
3bb.co.th. 5 NS ns2.3bb.co.th.
Authority
The response has no authority records.
Additional
ns1.3bb.co.th. 5 A 110.164.252.138
ns2.3bb.co.th. 5 A 110.164.252.139
ns1.3bb.co.th. 5 AAAA 2403:6200:5:1:0:0:0:39
ns2.3bb.co.th. 5 AAAA 2403:6200:5:1:0:0:0:3b
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0