3G can be turned off, as with Wi-Fi. If 3G is off, it reverts to EDGE. As I understand it, GPS does not have an on\off switch, rather is just disables itself when not in use, like if you get out of the Maps applet.
I am basing the GPS auto-disable off of what I have heard, I did not hear this from an official source, so if I am wrong I'll ask someone in the know to chime in1!