gistfile1.txt

\documentclass{article}
\begin{document}
\tableofcontents
\appendix
\section{alpha}
\section{beta}
\section{gamma}
\subsection{alfa}
\subsection{bravo}
\begingroup\def\addcontentsline#1#2#3\relax\subsubsection{FIRST}\endgroup
% this doesn't show up at all in the doc!
\subsubsection{second}
\subsubsection{third}
\subsubsection{billionth}
\subsection{charlie}
\subsubsection{first}
\subsubsection{second}
\subsubsection{third}
\subsubsection{billionth}
\end{document}