Generation of third harmonic plasma emission by coalescence of second harmonic radiation with a plasma wave is more favourable than by direct coalescence of three plasma waves. The predicted ratio of the intensities at the third (III) and second (II) harmonics is IIIIω/IIIω ~ ωLWlL/Nmc3, where ωL is the plasma frequency, Wl is the energy density in plasma waves, L the linear dimensions of the source and N the ambient electron number density. The intensity at the second harmonic is where LN(∼1010 cm at ωL ∼ 2π × 108 s−1) is the typical dimension of coronal inhomogeneities.