How can I modify a USB cable with a voltage-divider to provide 3V?
It is possible, but you need to remember that the load is going to have an effect on the voltage. 

Take a look at this circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

Nice and simple yes? Vin is going to be divided by 2, because both R1 and R2 are the same. Now let's add a load on there and see what happens.....







simulate this circuit

Hmm.... As you can see, there is now something in parallel with R2. This is going to affect the value of Vout. So... What can we do to stop that happening? Well, we are going to need a buffer to make sure the voltage stays constant!







simulate this circuit

Great! We now have a constant voltage. But hang on, there is more to consider.... What if the load requires more power than your buffer can handle? Remember, an op amp has a low impedance output (an ideal op amp has a zero impedance output), so will not be able to operate under heavy loads. Well, we can now use a circuit like this:







simulate this circuit

This is the basic schematic to the majority of linear regulators. All you need to do is use the negative feedback to control the output voltage. 

If you didn't want to do all that, just use a 3V regulator from any electronics manufacturer that fits your specs