  We revisit the calculation of third order \acp{SIGW} and extend it from a
monochromatic primordial power spectrum to a more general log-normal one. We
investigate the impact of third order SIGWs on \ac{SNR} of \ac{LISA} and
\ac{PTA} observations, and find that third order SIGWs significantly contribute
to the total energy density spectrum of \acp{GW} in high-frequency region. For
a primordial power spectrum amplitude of $A_{\zeta}=10^{-2}\sim 10^{-1}$, the
effects of third order SIGWs lead to a $40\%$ to $400\%$ increase in the SNR
for LISA. Additionally, our PTA data analysis reveals that third order SIGWs
diminish both the amplitude $A_{\zeta}$ and the peak frequency $f_*$ of the
primordial power spectrum.
