NS records and serial numbers

[rob at lexiconn.com]

A few questions:

- If a domain name has 3 nameservers, does the zonefile *have* to 
have all 3 listed as:
IN NS ns1.
IN NS ns2.
IN NS ns3.
I ask because some of our zonefiles on some nameservers do not 
list all 3 (We added a 3rd nameserver but some old records did not 
get updated). What problems could this cause?

- If using 3 master nameservers for 1 domain, does the serial 
number have to be the same on all 3 nameservers, or is it 
namserver specific?

Thanks

Rob