Can an arduino do PWM at 3 MHZ?
No... The default PWM frequency is ~500Hz. And, you probably don't want PWM anyway.
I assume the thing runs off a square wave, and there are easier ways to generate a square wave. I'm not sure if an LM555 can go to 2MHz, but I'm sure there are oscillator/multivibrator chips that can. You'd also need to boost the power with a MOSFET or something.
You can also buy a complete fogger/mister
with the oscillator built-in. One nice thing about these (besides being built and ready to go) is that the oscillator & electronics are already enclosed in metal so you aren't "broadcasting" high-power RF signals. I've got one of these things... I forget the power rating... probably medium power... I paid less than $100 USD for it.... I tried it out in the bath tub and it generated enough fog that you couldn't see the water, but not enough to fog-up the bathroom... Dissapointing... I think the droplet size is large compared to a regular fog machine and I'm guessing it falls to the ground before it can be spread-around.