Bluetooth and 3G are rather different technologies which serve a different purpose.
Bluetooth, is a low-power, low-cost, wireless connection between electronic devices - the analogy is that of a wireless version of USB. A bluetooth device can interface to your PC, or other bluetooth enabled device.
For example with a bluetooth enabled cellular telephone (e.g. Nokia 6310) and a laptop/pocket PC with bluetooth, you could connect to the internet using the cellular network without physically connecting (or even ensuring line of sight) between PC and telephone. The same principle with a bluetooth, telephone headset - make and receive calls, while the telephone remains in your briefcase.
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3G (or UMTS) is an evolution of the cellular network system. It provides a cellular telephone network with high bandwidth to support techniques such as videoconferencing, instant multimedia messaging, broadband access to public data networks (e.g. the internet). Although interworking with localised systems such as DECT allows a bluetooth type interconnection to devices in the immediate vicinity - the main thrust of UMTS is as a wide area, high mobility, public network.