The frequency of a radio wave is inversely proportional to its wavelength, so the longer the wavelength, the lower the frequency. The formula for calculating the frequency of a radio wave is:

```

f = c / λ

```

where:

* f is the frequency in Hertz (Hz)

* c is the speed of light in meters per second (m/s)

* λ is the wavelength in meters (m)

In this case, the wavelength is 3 m, so the frequency is:

```

f = 300,000,000 m/s / 3 m = 100,000,000 Hz = 100 MHz

```

Therefore, the frequency of a radio wave with wavelength 3 m is 100 MHz.