The two most common ways to get a white laser are Argon-Krypton gas lasers (argon provides the blues and greens while the krypton provides the reds and yellows) or individual red, blue, and green wavelengths combined in the right proportions to give white. The latter is how laser projectors do it.
Yes. Use the relative brightness product of response to calculate individual wavelengths of light using average human reponse so that equal colours are mixed into a single beam. Its probably best done with some optics, such as dicroics and mirrors, although I have never actually built one.
Snell's law in physics states that white light, i.e. a mix of colours will not be possible because diffraction patterns will yield inconsistent beam patterns when passing through media, namely, glass or acrylic lenses. That is why you must collimate laser beams before you use dichros. It'll be a mess to do so though, 3 or more modules and lenses in perfect formation is rather difficult.