With the help of this simple circuit you will be able to convert any voltage between 0.9V and 1.6V to a fixed 3V voltage therefore it is very useful when you need to power equipments that require 3 volts but you have only a single 1.5V battery or a 1.2V rechargeable one.
As you can see below the boost converter circuit uses only 4 parts, 2 capacitors, one coil and the main part the LTC3525L-3 IC from Linear Technology. The IC is a high efficiency synchronous step-up DC to DC converter that can start up with an input as low a 0.7V and generates a fixed 3V output voltage. The nominal output current is around 65 mA therefore you must keep this in consideration if you want to power up devices that require higher currents.
You may use a switch between pin 1 and 3 if you want to turn ON or OFF this circuit.
If you cannot find the LTC IC then you can look for the APE2902Y5R-30 produced by Advanced Power Electronics. It also provides a fixed 3V output voltage and 100mA current. As you can see the schematic is not so different from the one above and the only additional component is the B140 Schottky barrier rectifier diode.