To calculate the beat frequency of three frequencies, such as 200 Hz, 206 Hz, and 209 Hz, the beat frequency is determined by finding the absolute differences between each pair of frequencies. The relevant equation is Fb = abs(F1 - F2). Possible combinations include calculating the differences between 200 Hz and 206 Hz, 200 Hz and 209 Hz, and 206 Hz and 209 Hz. The resulting beat frequencies from these combinations will provide insight into the overall beat frequency. Clarification on the term "possible combination" was requested, indicating some confusion about the calculation process.