adwsystems:
The Uno requires 7-12vdc. A boost circuit that will accept 3v and output 7v should work.
It would be more accurate to say that an Uno runs on 5v which can be supplied to the 5v pin or it can be powered by 7-12v supplied via the barrel-jack.
An Uno runs at 16MHz and an Atmega 328 will not run at that speed on 3.3v. However an 8MHz Atmega 328 on a breadboard or on stripboard will work very happily on 3.3v. I think it is possible to buy a 3.3v version of some of the smaller Arduino boards.
...R