When two waves of identical frequencies interfere they produce standing waves. These waves always appear to be standing still. Harmonics are the wave pattern of the frequencies.
The third harmonic pattern consists of three anti-nodes. Each loop is equivalent to one-half a wavelength, the length of the string is equal to three-halves of a wavelength. L=3/2 *lambda
L is the length, in our case : L=1.2m
lamba is the wavelength. So, lambda=L/3/2=2L/3
lambda=2.4/3=0.8
