The kindle 3g prefers to use WiFi, that is the default first choice. As long as you stay in WiFi range and have WiFi networks identified and setup on your kindle, the kindle will never switch to 3g when you have WiFi turned on, unless you go out of range of the WiFi routers. That is the only thing that will force your kindle to switch and use 3g. I do not think there is a easy way to disable the 3g function.
Only turn WiFi on when you are in a hotspot.