For a string fixed at both ends, in \(n^{\text{th}}\) harmonic:
\[
L=n\frac{\lambda}{2}
\]
For third harmonic:
\[
L=3\frac{\lambda}{2}
\]
So,
\[
\lambda=\frac{2L}{3}
\]
Distance between consecutive nodes is:
\[
\frac{\lambda}{2}
\]
Thus:
\[
\frac{\lambda}{2}=\frac{1}{2}\cdot \frac{2L}{3}=\frac{L}{3}
\]
Hence, the correct answer is the option corresponding to:
\[
\boxed{\frac{L}{3}}
\]