Now I have this:

SERVER1:

name server 1 (ns1)
name server 2 (ns2)
main domain

SERVER2:
subdomains for main domain on SERVER1

SERVER3:
subdomains for main domain on SERVER1

and now I have problem, when SERVER1 don`t work all subdomains on SERVER2 and SERVER3 don`t work because DNS is on SERVER1!

Now I want add new name server for MAIN DOMAIN (ns3) and I want use IP from SERVER2 or SERVER3 and when SERVER1 don`t work subdomains must work....

How I can setup that, I try to do setup it but every time when I check DNS`s on checkdns.net ns1 and ns2 work fine but for ns3 I have this error:
Error fetching SOA from ns3.main_domain.com

I hope that somebody can help to me, please tell to me all steps, on all servers I use cPanel...

Thanks.