Answer
$8.00$ Hz
Work Step by Step
To find the frequency of the sound heard by an observer, the Doppler effect equation is used. This states that $$f'=f_o(\frac{v \pm v_o}{v \mp v_s})$$ where $f_o$ is the original sound frequency and $v$ is the speed of sound. The two frequencies can now be calculated. $$f'_1=(440Hz)(\frac{343m/s}{343m/s+3.00m/s})=436 Hz$$ $$f'_2=(440Hz)(\frac{343m/s}{343m/s-3.00m/s})=444 Hz$$ The beat frequency is the absolute value of the difference between the frequencies, which gives $$f_{beat}=|444Hz-436Hz|=8.00Hz$$
