I'm assuming you want your iPad3 to be a hot spot (as in, broadcast a WiFi signal - using your 3/4G connection - so that other devices can log in to browse the web). With that assumption...
First off, you have to be on a plan through your carrier that allows you to set up the hot spot. Once you do, go to your Settings app on the main tab should be a line titles "Personal Hotspot." Again, without a carrier plan, this line won't even show. Turn the hotspot on so you are broadcasting and log into the WiFi signal on the other device. Oh, and this only work on 3/4G. If you are on WiFi, it won't work (or else your WiFi will turn off in order for the iPad to use 3/4G. You can't share a WiFi signal (which makes sense...).
Hope this helps. If I've made ghe wrong assumption, please clarify and we can then help with specifics.
Marilyn