Cutting 5 Volts back to 3 Amps

I have a battery operated Christmas Lights that run off 3 volts battery supply. I wouldn't mind wiring these up to be USB powered from my computer.
How can I transform the 5 volts from USB to 3 volts?

Thanks,
Jim

Start by finding out how much current the lamps use.
Without math, I can say, put 3 silicon diodes in series with the lights. That will use up a bit more than 2 volts.

I wouldn't. Use a 3V to 5VDC 1A plug-in cell phone charger instead. You probably have several old ones in your closet.

For a simple string of lights that likely draw less than 1/2A of current (you do have a multimeter capable of measuring the current that the lights draw from the battery pack, don't you?), I would just use a power resistor in series with the string. Say the charger puts out 5V. You want 3V at the string, so 2V is dropped across the resistor. If the string requires, say 0.4A, then R = E/I = 2/0.4 = 5Ω. The power dissipated in the resistor is P = IE = 0.4*2 = 0.8W, so I would get a 5Ω 2W resistor...

Thanks for the replies, I do have a multi meter thank you. I should know these calculations but it has been years since I have done any of this stuff.
I forget to mention they are only L.E.D.s wouldn't draw much.

Jim

Ok, measure current. Calculate R and P. There will be a quiz.