Refer to the figure shown below.
At the 3rd harmonic, there will be 3/2 wavelengths over the length of the guitar string. That is,
L = (3/2)λ
where
L = length of guitar string
λ = wavelength
Note that v = fλ, where
v = wave velocity,
f = wave frequency,.
Therefore
L = (3/2)λ
= (3/2)*(v/f)
= (3/2)*(125 m/s)/(170 1/s)
= 1.1029 m
Answer: 1.103 m