Let Vs be the input voltage.
The 3rd harmonic is the lowest order harmonic.
$({V_o}\left( {rms} \right) = \frac{{2{V_s}}}{{n \times \pi \times \sqrt 2 }})$
For fundamental component n = 1 and for the lowest order harmonic n = 3
$({P_3} = \frac{{{V_3}}}{{{V_1}}} = \frac{1}{3}\;or\;33.33\;\% )$