Stepping up 3V to 5V
The first solutions given are low power really, because I didn't know yet what the current needs were. A higher current solution answering Paul's actual needs is given at the end of this answer.

You don't say how much current you need from the higher voltage, but I'll presume it's for a power supply, say at least 100mA.
For this an SMPS (Switch-Mode Power Supply, aka "switcher") is usually used. SMPS basically can be divided in two groups: buck (or step-down) and boost (or step-up) converters. To go from a lower to a higher voltage you need a boost converter.  



The example shows a boost converter to get 3.3V from a pair of AA cells, but this can be easily adapted to a 5V output; all you have to do is change a resistor value.
So we see that a switcher needs a few more components than a classic linear regulator like the LM78xx, but it's not that bad.  



If you only need little current you may consider a charge pump voltage doubler. 



You can have then a LDO (Low Drop Out) regulator to go from the 6.6V to 5V. Keep in mind that the output voltage of a charge pump drops rapidly with increasing current drawn.

edit
If you need 3A as noted in your comment the above proposed solutions won't do. You can use a National Simple Switcher like the LM3478 or one of these Linear devices. National's Webench tool created a design with a BOM cost of USD 4.36, Linear tends to be a bit more expensive.



More:
Boost regulator selection guide at Linear Technology