How to share internet connection?

I have three desktop pcs. One of it connects to the internet via a ADSL modem which uses a LAN connection to the network card of the computer. The problem was, i could only connect one pc at a time to the internet as i have to share the ADSL modem(Riger DB102 ADSL Modem 2/2+) between the three pcs. To solve this, i bought a Switcher (Aztech 5port 10/100 Switch). I successfully created a workgroup where i can now easily share files over the network between the three computers. The problem is however, i still cannot get internet access on all three pcs at the same time. Only one seems to be able to connect while the other two can neither connect nor share the 1st pcs internet. Please advice. thank you.