Well it sounds like a normal wireless switch will give you the same problems as your ADSL modem/combo thing. The issue is that you aren't sharing the connection. There are plenty of ways to do this, the easiest (and least secure) of which is to use Windows Internet Connection Sharing or some proprietary software on the "main" PC and share that with the rest of the PC's via the WiFi. The most effort (and most secure) would be to get yourself a cheap old computer, install a Linux distribution like IPCop. IPCop is a flavour of linux that aims to provide a simple-to-maintain, secure firewall. Then you would plug your 3G modem into the IPCop box, and plug the IPCop box into your wireless switch (or ADSL/combo thing), making the IPCop box the "main" PC that has the direct Internet connection, and the rest of the PC's just clients that use the IPCop box as their gateway.
If this all sounds way too complicated to you, fear not! It's not really that bad (and there are PLENTY of resources on the web that will help you out, as well as plenty of forumites that I'm sure would be more than willing to help).
The first step to take here, is to decide exactly what kind of setup you would like, then to have a look around (or post here) to see if it's feasible, and if not, what the alternatives are. Personally, I would rather look at getting ADSL again (in the long run, it'll be more cost effective), but if that isn't an option, IPCop would be my second choice (can pick up a cheap PC that will do the job for less than R500 these days). Then comes proprietary software, and finally, Windows ICS.
Edit: When I said "normal wireless switch" that's exactly what I meant. A 3G router should do the trick, but in my opinion, getting one wouldn't be worth the money (unless you were sure you were going to be using 3G as your main WAN connection for several years).