chapter{something}
section{1.1}
subsection{1.1.2}
subsubsection{Number 1} label{sssec:num1}
subsubsection{Number 2} label{sssec:num2}

documentclass{report}
setcounter{secnumdepth}{3} % default value for 'report' class is "2"
begin{document}
chapter{Something}

Here are cross-references to subsubsections ref{sssec:num1} and ref{sssec:num2}.

section{A section header}
subsection{A subsection header}
subsubsection{First subsubsection} label{sssec:num1}
subsubsection{Second subsubsection} label{sssec:num2}
end{document}