Bilal,
The setup you want is not that difficult and fairly easy to setup, even for people that are new to networking. Your question is also something nearly everyone here can answer, so its in everyones interest to post questions that are not directed to one person.
Thought I do not have the comfort to explain step by step how you can setup such a network, I can give you a few clues or hints as to what you need to do, and if anyone can fill in the gaps, please feel free to do so!
To begin, let's clarify one thing. Your originally spoke about creating a network with 6 pcs, and then you talk about having 3 pc's on three networks, thats a total of 9 pc's - or hosts if you would like.
So I'm assuimg that the Win2k Server will be attaching its self to each network, hence the number of 9 hosts (6+3).
To keep things simple, there are 2 ways you can setup the above network(s), assuming we are going to only use Class C networks:
1) Use a different Class C network address for each of your 3 networks.
2) Use one Class C network and subnet it so you split it into smaller 'peices'.
The easiest approach is the first method.
So, taking that in mind, you decide the Class C networks you want to use in each one of the 3 networks, e.g
Network No.1: 192.168.1.0/24
Network No.2: 192.168.2.0/24
Network No.3: 192.168.3.0/24
So you have 3 nice logical networks now on the same physical network ( your hub).
All you do now is make the Windows 2k server part of each network by binding an IP Address to each network interface card the server has. Once this is done, the server will automatically update its routing tables and all you need to do is have the windows 2k server set as a "gateway" for each of your workstations and also enable IP Routing on your server (which I cant remember how to do right now).
I would suggest you also read the 'Communications between subnets" section which will greatly help you understand how similair setups work.
Cheers,