Network adapter in each PC, connect with "crossover" CAT5 ethernet cable, set up Internet Connection Sharing on the PC connected to the broadband modem.
That's it in the simplest form. The 'host' PC obviously has to be running for the 'client' to have internet access.
When (if) you want to add an additional PC, you need to include, ideally, a router in the system so that any PC can acees the internet independently, without a 'host' PC having to be running. Otherwise, you need a hub/switch to allow three PCs to be networked and use the 'host' PC's shared internet connection.
With a router, you need to be able to connect to the broadband modem by ethernet, NOT USB, so if your connection is currently ADSL connected by USB, this would have to be replaced with an ethernet modem, or use a router with built-in ADSL modem.