Thanks for answering HBP!
Don't know if LM317 is a 3-volt regulator, read the datasheet and did not find any references to this.
Anyway, I think I didn't explain things very clear.
Power suply that feeds the Led delivers 4V/0.75A , so I would like to regulate these 4V in a range from 1,2v to 4V in order to obtain different intensities of led light depending on the rotation of the potentiometer. (I did not build this power supply, and must use this one).
However, using the LM317 I get a range from 1,2v (pot at max counter clockwise) to 2,6v (pot at max clockwise) and would like to get closer to the 4v to go from barely no illumination to full led illumination.
All voltage ranges are being measured from LM317 output to ground.
Hope it's clearer now, should've taken more english lessons!!