Hi Conor,
If you are talking about supply voltage to run the sensors, then yes, there are both a 5V rail and a 3.3V rail that is powered all the time. You can't pull a lot of current, but for a few sensors you should be OK.
The other thing is the communications: On the Galileo, you can change the logic levels, but all I/O pins will use the same, you can't have some pins that does 3.3V and others at 5V, but for a prototype, you might be able to run it all at 3.3V, if not you will need level translation on the communications pins (eg. something like https://www.sparkfun.com/products/11771 or https://www.sparkfun.com/products/12009)
/Thomas