So what i have:

3 networks connected to a router. In the 1 network I have s Slackware DHCP server for the same network. Now I have to configure the DHCP server to give addresses to the other 2 networks and ofcourse to do that through the router

The router is a Cisco product with Cisco IOS

I now that I clould configure the router to do that for every of the networks or to move the DHCP server in the other networks, but my assignment is to configure the router to give addresses through the router.... Does anyone have an idea how am I supposed to do that ?

I am thinking of the idea to configure the 3 pools in the DHCP server and to make the router helper for that.... but I am not so sure that I could be so easy...

Yes, that's really quite simple, just define additional scopes on the dhcp server exactly as if they were locally connected, and then add an ip helper under IOS - "ip helper-address ip.of.dh.cp" under which interface you want to enable it on.

I know that slackwarehelp.org has an guide for it.
But the firewall in the guide isn't too safe so you might want to make it more secure.
http://slackwarehelp.org/viewtopic.php?f=11&t=14

not a good idea!

Take a look at what this command actually does:
http://www.cisco.com/en/US/docs/ios/...html#wp1169356
Why?
1. It's an interface command, but such configurations are generally put in global configuration. If you think about it, what if you have 20 networks, would you configure for 20 interfaces? what about 50?
2. This command will forward ALL broadcasts to this address and you don't want that really.

This is what you need to use:
http://www.cisco.com/en/US/docs/ios/...html#wp1168014

This is what I would do:
The configuration you are looking for is defining 3 VLANs each for each subnet.
The port that connects to the DHCP server will have 3 subinterfaces each with an address from each of the subnets. For instance you would have 10.0.1.0/24, 10.0.2.0/24 and 10.0.3.0/24 with fa0/4 (for example) going to the DHCP server.
Then fa0/4.1 will be 10.0.1.1/24, fa0/4.2 will be 10.0.2.1/24, and fa0/4.3 will be 10.0.3.1/24. fa0/4 goes to the DHCP server which also has subinterfaces defined for each of the subnets. Make sure all networks are in the routing table, that broadcast forwarding is allowed (or use the ip helper / ip dhcp-server commands) and you are good to go.

It's perfectly fine idea thanks. I'd limit the helper ports to only forward 67, but as the OP does NOT have 50 interfaces, it's fine. And where did VLAN's come into the equation? You can't assume that there are tagging switches involved, and even if there are then that's nothing to do with DHCP in the slightest. and erm... my advice is apparently exactly the same as yours once you take away the bits that weren't asked about in the first place.