Step 1: Free Your Phone
There are lots of reasons to jailbreak your phone. Skype and SlingPlayer over 3G is just one of them. Luckily, we've got a comprehensive guide to easily jailbreak your iPhone 3G (or iPod touch). Follow it.
Step 2: Trick Your Phone
Welcome back! If you're just interested in using Skype, SlingPlayer and Fring's VoIP over 3G we're already at the end—search for Tricker Three3G in Cydia, install it, and you're good to go. SlingPlayer, Skype and Fring will think they're running on Wi-Fi, no matter what connection you're on. Right now, Tricker Three3G only fools those three apps. There's a more involved process if you want to fool other apps too.
Step 2b: The Long Way
In Cydia, the app you want to search for and install is VoIPover3G, which tricks any app into thinking a 3G connection is actually Wi-Fi. You also need to grab OpenSSH or DiskAid and the latest version of MobileSubstrate. From there, you have to tap into phone and install some custom text files for every app that you want VoIPover3G to trick. But you do have the power to trick any app with it, unlike Tricker ThreeG. If you're completely new to the game, I'd start with this guide to getting Skype to work over 3G and then just substitute the final steps for the relevant Sling parts from this guide. They're laid out pretty well, so fairly easy to follow. [ModMyi, iHackintosh, iPhone Download Blog]