Yes, three narrow-band red, green, and blue sources can be mixed so the eye perceives them as white, similar to how some displays and RGB LEDs work. By changing the relative intensities, the perceived white point can shift warmer or cooler.

However, this is not the same as true broad-spectrum white light. Three lasers produce only three very narrow wavelengths, not the continuous mix of wavelengths found in daylight, tungsten, or other full-spectrum sources. So while it may look white to the eye, a spectrometer would show three sharp spikes, and color rendering on real subjects may be poor or unnatural.

Because color temperature in Kelvin is really tied to broad-spectrum light sources, applying a Kelvin value to RGB laser light is only an approximation based on visual appearance, not a true spectral match.

So the practical answer is: you can create something that appears white, but you cannot reproduce genuine full-spectrum white light with only three lasers.