at a high level,

- pick 2 ports on each switch to be 'unsecured'. (for example ports 1 and 2)

- put these ports in a different different untagged vlan (for example vlan 100)

- set the port vlan of these 2 ports to be vlan 100 also.

- connect switches like this: internet->switch 1 port 1, switch 1 port 2-> switch 2 port 1, switch 2 port 2 -> switch 3 port 1, switch 3 port2 -> server unsecured network interface

- this will connect your unsecured internet to your server.

- you could now also connect the switches like this: switch 1 port 16 -> switch 2 port 15, switch 2 port 16 -> switch 3 port 15, switch 3 port 16 -> server SECURED  network interface

- this will connect all your clients to the secure interface of the server.

- you will have to run some kind of routing firewall on your server and configure it to route between the network interfaces if the clients are going to be able to access the internet through it

Can't we get both connections on one port and then it only react on the tags.

Example.

On the first switch, there the servers lan is going in to, can't i set that port to tag with 100.

And then set the port the servers wan is going in to, to tag with 80.

Then set the these to connection to go though one port on the first switch or do i have to do it with two different ports?

Sure.  You can use a single port to connect the switches.  Here is an example using vlans 100 and 80:

Thanks for your reply, i will try it on monday.

I have got it to work.

Thank you so much!

I just have a last question, if all my port are PVID 80/100 how do i contact the switch?

Unfortuanately, this switch is only manageable on vlan 1.  To answer your question, you cannot change all your PVIDs to 80/100 and still be able to manage the switch.  You will have to have at least one port that is a member of vlan 1 for management.