though i am not an expert but these are my suggestions----
a modem can connect to a computer through either USB or LAN interface.you can connect 2 computers to same modem by connecting 1 computer to USB & another to LAN.in this arrangement both computers can use file sharing & internet too.however if you want to connect both the computers simultaneously to net you must set your modem in pppoe mode.
besides these two computers if you want to connect a laptop too you have to add a wireless router to this network.then you have to setup the adsl modem in bridge mode & wireless router in pppoe mode.in this network too you can use file sharing & can connect to net simultaneously from computers/laptop.
basically ICS is not recommended for such networks where you want to connect to net simultaneously from,say,a laptop while 1 computer is already connected.also if you planning to add wireless router then again ICS is not recommended.pppoe mode is the preferred mode for such networks.you don't have to worry about anything because pppoe setup can handle additional computers/laptops too if you decide to add them in future.