Wifi and 3G (mobile/cellular connection) are two different ways to connect your Kindle to the Internet. Are both not working?
Have your tried restarting your Kindle. To do this, slide the power button to the right, hold it there for atleast 15 seconds and then release it. PS: I don’t have a Kindle 3 handy to test the above method of restarting. So let me know if it worked else I will try to borrow it from one of my friends.