Connecting arduino to power another device
Since it's battery powered exact voltage is maybe not so important, and you may be able to power it from a 3.3 V supply. Let us know what the device is and we can further advise you.  

Anyway, I mention the 3.3 V because the Arduino has a 3.3 V regulator on-board, though it can only supply limited power: a current of 50 mA maximum (Uno). Since you talk about batteries this may be sufficient, but if you need more power I would bypass the Arduino's voltage regulators altogether, and go directly from the Arduino's power input to the 3 V. At high currents and possibly high input voltage a linear regulator will lose much power, so that a switching regulator may be more fit. 

So again, tell us what you want to power and we'll tell you more.