Christmas Tree LED lights

Return them and purchase some that already have that function.

Seriously, it's far more trouble than it's worth to do something you can purchase for pennies (figuratively).

But if you really want to do this yourself then yes, voltages and currents will be needed although 'generically' you will need either a PWM controller (for fading) or a simple astable switch for flashing.

 

Most likely, the lights are wired in series and run directly from the line voltage. Unless you have the experience, working with line voltage is dangerous.

But we can probably determine that.

Is there a box between the line plug and the lights? If not, then they are running at the line voltage.

Bob

 

With a supply that is only 3V then an ordinary Mosfet will not turn the LEDs on and off, even a "logic level" Mosfet will not work. Some small Mosfets used in radio controlled toys work from 3V.
Then make a Cmos oscillator to turn the Mosfet on and off.

A PWM circuit can fade the little Mosfet without burning it out.

 

Here is one that has an on resistance of 147mΩ at 2.5V and can carry 2A. Perfectly suitable for 3V switching.

http://www.mouser.com/ds/2/308/MCH3481-D-775525.pdf

Bob