Can the iPad 3G only use 3G and Wifi, or can it use Edge or 2G (the little blue dot on iphone)?
"3G + WiFi" is just a commercial tag line.
the 3G part stands for internet access throughout a mobile connection, usually using a SIM Card.
The full range of connections on a "3G" model can be found in the Technical Specifications of the product, in this case:
The iPad uses the same mobile chip set as the iPhone and cam (depending on your data provider) use Edge (denoted by an E) or GPRS (seen as a circle).
O2 in the UK definitely does.