The quick easy answer is Radio Shack.
Go there and buy an assortment of resistors, pretty much any of the standard resistor assortment packages they have are fine since you are talking low amps, low voltage. Grab your DMM (digital multimeter) and 4 d cell batteries. Electrical tape the 4 batteries together flashlight style and tape a red wire to the positive end and a black to the negitive end of your battery stack. You now have 6 volts. Try different resistors attached to the red wire and the resistor to the DMM red lead and the black wire to the DMM black lead. One or two of the resistors will get you very close to 3 volts. that is the elementary method for you.
The expert way is to read the color bands on the resistors and find the one that completes the formula to reduce 6 to 3 volts.
The way I would do it is turn my bench power supply to 6 volts and get my Fluke DMM and try a few of the assorted resistors I have until one gets me from 6 to 3 volts. More than one resistor can be used in line.
I would try a resistor in the range between 100 ohm and 800 ohm is I remember correctly. The DMM will tell you right way if it is even close.
Good luck

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Christopher Henkel
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