Hmm, sounds like you are in an area with no 3G coverage, the iPad should always revert to the best connection available, it's an automatic process and can't be user controlled.
As Gabriel says, the iPad will always move to 3G from EDGE if there is a 3G service available. You might need to contact your service provider and see if there's a network problem. To see if the problem really is with your iPad try to find another mobile device user on the same network and see if they can receive 3G.