>My question is if all 50 users need to access all 3 servers, do I need
to buy 3 copies of windows 2003 server and 50 CALs per server, so I need
150 CALs or can I buy 50 CALs for my 50 users?

The answer depends on whether Windows Server 2003 is licensed “per
user/device” or “per server”. For both licensing modes, you may choose
to license either users or devices.

If you choose Per User/Device licensing, then you need one Windows
Server 2003 CAL for every user OR device.

If you choose Per Server licensing, then for each of your three
servers, you need a number of Windows Server 2003 CALs equal to the
maximum number of concurrent authenticated sessions that server will
host.

I think this overview is pretty clear:
http://www.microsoft.com/windowsserver2003/howtobuy/licensing/caloverview.mspx

HTH,

Zac Mutrux