Convert from 3 V to 5 V
You may want to use a boost converter which is a device that can step up voltage.

You can get them from eBay for around $US 2.



As Chris Stratton pointed out in a comment, the boost converter may run your batteries down quite quickly. Let's say you need 200 mA for your Uno, sensor and LEDs. At 5V power that is 1W of power (0.200 * 5).

However to get 1W from 3V you would consume 333 mA (0.333 * 3). Then take into account that the converters are not completely efficient. Let's say the converter has 90% efficiency. Then you need to multiply the current by the inverse of the efficiency, that is: 0.333 mA / 0.9 = 0.37 mA.

Another factor is the battery type. If you use NiCad batteries their nominal voltage is closer to 1.2V than 1.5V, especially after you have been using them for a bit. Ditto for NiMh batteries.

Reworking the above for 2 * 1.2V (2.4V) we see that the current will be:

1W / 2.4V = 0.4167A
0.4167A / 0.9 = 0.463A (allowing for 90% efficiency)


Now it potentially draws close to half an amp when active. Your batteries might not be able to handle that for long.