|We would need to know more about your setup to help you. |
What are the IP addresses of the 3 servers?
Are they all on different subnets or within the same one? If the same subnet, then you need to know that all clients would be able to connect to all 3 servers.
How is your DHCP setup and which server(s) is it running on?
You say 20 thin clients are supposed to connect to each server. How have you set it up so that each group of 20 knows which server to connect to?
Lets start with those questions and go on from there.