Hello Pradeep,
Different sites, 1 domain?
As stated, read the manual, follow instructions. (I'm going from Exchange2007 install)
In short.
You have 1 ev-server in each location
You have 1 CAS server in each location
configure policies (OWA)
create ExchangeServers.txt file in c:\program files\enterprise vault folder, having the ip-adresses of the cas-servers (each ev-server all 3 ip-adresses)
run the owauser.wsf script with the parameters as stated in manual.
restart adminservice, sync users.
install extentions on CAS-server (pick the right ones (x86 or x64), and it will work.
For Exchange2003 it is little more work, but it will work. Read manual as Wayne and Tony indicated. If not comfortable, ask more. Do remember that EV is not a product you can install/configure and use without RTFM!! It is complicated, and 'clicking around' most likely will get you in trouble, or will not live up to expectations.
Regards. Gertjan