Once you know the location of the first peak (fundamental frequency or what you appear to be calling the 1st harmonic), you then look at the signals at 3, 5, and 7 times that frequency. Depending on the resolution and the relationship between the sampling rate and the signal frequency, you may need to combine the values in several adjacent bins to get all the energy in each harmonic. Search for information on "spectral leakage" for the reasons.
Lynn