What is the wavelength of light that has a frequency of 3 MHz?

Wavelength = (speed) divided by (frequency)

Speed of light, radio, infrared, x-rays, etc. = 300,000,000 meters/sec

Wavelength = (300,000,000 / 3,000,000) = 100 meters.

You can't see that wavelength, and it's not usually described as "light".
It's in the region of the electromagnetic spectrum known as "radio".