The easiest way would be to install an inexpensive USB ethernet adapter on the laptop (they can be had for as little as $2-3 on eBay) and connect it to the WAN port of the router. Then enable ICS on the 3G modem and share it w/ the ethernet connection.
Alternately, it’s possible to eliminate the router completely and use Connectify to reconfigure your laptop’s wireless adapter as a wifi hotspot and share the 3G modem.http://connectify.me
So it really comes down to whether you have a specific need to use the WRT54G (maybe you want all the features it has to offer; firewall management, port forwarding, static IPs, DMZ, etc.), or whether you’re just trying to use it for its AP. In the latter case, the use of Connectify will suffice as a replacement for the wireless router.