\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{calc} 

\makeatletter
\newcommand{\gettikzxy}[3]{%
  \tikz@scan@one@point\pgfutil@firstofone#1\relax
  \edef#2{\the\pgf@x}%
  \edef#3{\the\pgf@y}%
  \pgfmathparse{#2/28.3465} \let#2\pgfmathresult;
  \pgfmathparse{#3/28.3465} \let#3\pgfmathresult;
}
\makeatother

\begin{document}


\begin{tikzpicture}
   \coordinate (O) at (0,0,0);
   \coordinate (i) at (1,0,0);
   \coordinate (j) at (0,1,0);
   \coordinate (k) at (0,0,1);
   
   \draw[-latex] (O) -- (i);
   \draw[-latex] (O) -- (j);
   \draw[-latex] (O) -- (k);
   
   \foreach \p in {i,j,k}
   {
      \node at ($(O)!1.1!(\p)$) {$\underline{\p}$};

      \gettikzxy{(\p)}{\x}{\y}
   
      \node at ($(O)!1.5!(\p)$) {$\underline{\p}=(\x,\y)$};
   }
\end{tikzpicture}


\end{document}