Triple Approx Symbol
Maybe like this?
\documentclass{article}
\newcommand{\tripprox}{\setbox0\hbox{$\approx$}%
\mbox{\makebox[0pt][l]{\raisebox{0.48\ht0}{$\approx$}}$\approx$}}
\begin{document}
\tripprox $\tripprox$
\end{document}
Not an answer, more for the unicode curious and the fonters -
There's a triple tilde mathematical operator in unicode (at U+224B) which, converted to an accent, in the various math fonts looks like this (if I've done it correctly - it's totally experimental):
\documentclass[12pt,varwidth,border=6pt]{standalone}
\usepackage{xcolor}
\pagecolor{red!3}
\usepackage{unicode-math}
\usepackage{accents}
\setmathfont{XITS Math}[Colour=blue]
\setmathfontface\masana{Asana Math}
\setmathfontface\mdejavu{DejaVu Math}
\setmathfontface\mtgdeja{TeX Gyre DejaVu Math}
\setmathfontface\mpagella{TeX Gyre Pagella Math}
\setmathfontface\mbonum{TeX Gyre Bonum Math}
\setmathfontface\mschola{TeX Gyre Schola Math}
\setmathfontface\mtermes{TeX Gyre Termes Math}
\setmathfontface\mlatin{Latin Modern Math}
\setmathfontface\mcambria{Cambria Math}
\setmathfontface\mfira{Fira Math}
\setmathfontface\mfreeserif{FreeSerif}
\setmathfontface\mlibert{Libertinus Math}
\setmathfontface\mnoto{Noto Sans Symbols}
\setmathfontface\mqui{Quivira}
\setmathfontface\mstixtwo{STIX Two Math}
\setmathfontface\mstixgen{STIXGeneral}
\setmathfontface\msymbola{Symbola}
\setmathfontface\mgaramond{\detokenize{Garamond-Math}}
\newcommand\mfsize{\Huge}
\setmainfont{Noto Serif}
%Define \ttilde in unicode-math-table.tex:
%\UnicodeMathSymbol{"0224B}{\ttilde }{\mathaccent}{triple tilde accent}%
\newcommand\themassym{\accentset{\ttilde{}}{}}
\begin{document}
\section*{Sampling {\mfsize $\themassym $}}
\vspace{24pt}
\begin{tabular}{rccl}
XITS Math & \mfsize $\themassym$ & \mfsize $\mcambria \themassym$ & Cambria Math \\
\ &\ & \ & \ \\
Asana Math & \mfsize $\masana {\themassym}$ & \mfsize $\mfira \themassym$ & Fira Math \\
\ &\ & \ & \ \\
DejaVu Math & \mfsize $\mdejavu \themassym$ & \mfsize $\mfreeserif \themassym$ & FreeSerif \\
\ &\ & \ & \ \\
Tex Gyre Bonum Math & \mfsize $\mbonum \themassym$ & \mfsize $\mlibert \themassym$ & Libertinus Math \\
\ &\ & \ & \ \\
Tex Gyre DejaVu Math & \mfsize $\mtgdeja \themassym$ & \mfsize $\mnoto \themassym$ & Noto Sans Symbols \\
\ &\ & \ & \ \\
Tex Gyre Pagella Math & \mfsize $\mpagella \themassym$ & \mfsize $\mqui \themassym$ & Quivira \\
\ &\ & \ & \ \\
Tex Gyre Schola Math & \mfsize $\mschola \themassym$ & \mfsize $\mstixtwo \themassym$ &STIX Two Math \\
\ &\ & \ & \ \\
Tex Gyre Termes Math & \mfsize $\mtermes \themassym$ & \mfsize $\mgaramond \themassym$ & Garamond-Math \\
\ &\ & \ & \ \\
Latin Modern Math & \mfsize $\mlatin \themassym$ & \mfsize $\msymbola \themassym$ & Symbola \\
\end{tabular}
\end{document}