In a network that contains 3 servers and 100 workstations, where is the best place to install an Anti-virus program?

To be more secure install in all the 3 Servers, if you want to install in only one system then install in your main server ..

Use Symantec endpoint protection Antivirus install all the 3 Servers.

Use Symentic Antivirus install into to your main server after that its show you how many machine are communicating with this server if all the machine are communicating you just push to install the antivirus into all you workstation 

Install End-point security application on all the workstations as those are the most vulnerable points of compromise. This will also help us to detect any abnormal behaviour in any of the machines and isolate it when required. 

sever is a computer that controls other computer on network. you can install antivirus in your sever(all your sever) so that all information are safe. and you can send it to other worksation. you can use antivirus called "zone alarm free antivirus"