You've left out quite a lot of detail here. What version are the DC's and what version is the exchange server?
With regard to the "3 DCs" that don't exist, that sounds like somebody either didn't do a proper removal or the servers physically imploded.
You can checkout this post, found using google:
»exchangeserverpro.com/how-to-use ··· nt-shell
There is also this tip, for "supposedly" setting up the DC's and their DNS server setups and network settings as well. These are all good tips even if just for proper redundant setup of the DC's on your network, irrespective of the current Exchange server issue.
"Do you have two DCs in the SAME domain? Please check that they are two DCs for the same domain.
If it is the case then check the following:
Make sure that each DC has one IP address in use and one NIC card enabled (All other NIC cards should be disabled): Multihoming is not recommended for DCs
Make sure that each public DNS server in use is set as a forwarder and not in IP settings of your DCs
Make sure that each DC is also a DNS and GC server
Make each DC points to the other one as primary DNS server
Make each DC points to the its private IP address as secondary DNS server and 127.0.0.1 as third one
Once done, run ipconfig /registerdns and restart netlogon on both DCs. After that, please delete any DNS records for your DC which is obsolete and still in your domain DNS zones.
Now, make your Exchange server points to both DCs as DNS servers: One as primary and the other one as secondary. Same thing for client computers."