How to create RedHat Linux machine as router

Hello all!

I'm novice in Linux (I use RedHat) so I have a lot of problems
So... I need to create a router like this:

i-net
|
router
eth0(DHCP)
_________
computer1 --eth1| |eth2-- computer2
---------
eth3
|
|
computer3

eth0(DHCP)internet
eth1 IP 192.168.1.0
eth2 IP 192.168.2.0
eth3 IP 192.168.3.0

computer1, computer2 and computer3 should not see each other.
so can any one help me to find solution to solve this problem. Which
file I need to modify and which information I need to write in this
files.
And how looks computers1,2,3 network configuration
(IP,DNS,Gateway,Subnet mask).

Best regards and thanks to all!

With NAT configuration,

--
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FreE wOrlD = FrEe cOdE
By NeCrOS.
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