I'm currently sharing internet from an USB dongle 3G Modem to wireless connected computers with no extra program. It's enough to activate the Internet Sharing function in the System Preferences-> Sharing menu.Unfortunately the activation process is a little complicated:
1 - Airport must be active (System Preferences -> Network): on my MAC when the 3G connection is ON the Airport is usually disabled due to a behavior of the 3G control program (it creates a new Network Location on each connection)
2 - an ad-hoc wireless network must be created by means of the Airport Options button in the Sharing panel, PCs must connect to this network
3 - sometimes is needed to uncheck Internet Sharing and then re-check it in order to see and click a Start button, only after this I.S. is running
A further issue is that the ad-hoc wireless network does not allow to share files, printers and others. As a matter of facts is not possible to use a pre-existing network given by a wireless access-point or router (unless I'm mistaking something, corrections are welcome). This is not true when computers are connected via ethernet, here everything is working.