Would it be possible to get a "white" lighthing of a subject by using 3 different lasers (R, G and B)
If so, which wavelengths and proportions should be used ?
No. "White" light contains all wavelengths of light - the Kelvin temperature just affects the proportions of each wavelength. Three lasers will have just three specific wavelengths so can't possibly reproduce the full spectrum of light that is in white light.
You can get white LED lights that like monitors do actually create white from combinations of very narrow bands of R, G and B light - we perceive it as white of a certain "Kelvin" balance, but if you use a spectrometer, you will see spikes.
The same goes for fluorescent lights:
Compared to old light bulbs and halogen the benefit is that they don't emit IR light, ie. waste so much power on heat, which is why we see them as brighter at lower watts. However, cameras will see them as really dim, because the integration of the sensor's sensitivity curve times the emitting source curve ends up rather small, with all those spikes.