I think you need to create your own macro, something like:
\documentclass{beamer}
\newcommand{\mytimeditem}[1]{\only<+>{\item #1}\only<+>{\item #1}\only<+>{\item #1}}
\begin{document}
\begin{frame}
\begin{itemize}
\item<+-> item 1
\item<+-> item 2
\mytimeditem{item 3}
\item<+-> item 4
\item<+-> item 5
\item<+-> item 6
\end{itemize}
\end{frame}
\end{document}

If you want to leave a blank line where "item 3" was, replace the last \only into \mytimeditem with \onslide
Second version:
\documentclass{beamer}
\newcommand{\mytimeditem}[1]{\only<+>{\item #1}\only<+>{\item #1}\only<+>{\item #1}\addtocounter{beamerpauses}{-2}}
\begin{document}
\begin{frame}
\begin{itemize}
\item<+-> item 1
\item<+-> item 2
\mytimeditem{item 3}
\item<+-> item 4
\item<+-> item 5
\item<+-> item 6
\end{itemize}
\end{frame}
\end{document}

A more generic version
With expl3 (code from @Joseph Wright answer) and calc
\documentclass{beamer}
\usepackage{expl3}
\usepackage{calc}
\ExplSyntaxOn
\cs_new_eq:NN \Repeat \prg_replicate:nn
\ExplSyntaxOff
\newcommand{\timeditem}[2]{
\Repeat{#1}{
\only<+>{\item #2}
}\addtocounter{beamerpauses}{-#1+1}
}
\begin{document}
\begin{frame}
\begin{itemize}
\item<+-> item 1
\item<+-> item 2
\timeditem{4}{item 3}
\item<+-> item 4
\item<+-> item 5
\item<+-> item 6
\item<+-> item 7
\end{itemize}
\end{frame}
\end{document}
Now the relative number of repetitions can be specified on the fly and the number of pauses is automatically adjusted.

\item<+-.(3)> my item. The topic is intensely covered by the great answer of Joseph to Relative overlay specifications in beamer, which appears to be a duplicate of this question. Please take a look at it, if you agree we could close this question. – Daniel Oct 11 '14 at 19:49\itemlines using a relative spec. That's related to the link suggested by Daniel but not a dupe. Could you clarify the question? If I'm right, there is a solution related to my answer in the link, but it's a not quite the same. – Joseph Wright Oct 12 '14 at 20:52mwewhat I exactly wanted is the syntax to deal with an overlay between current position and 3 slides later. Then I cas use it with\only,\visible,\invisible... it depends. I just looked for the syntax, (Daniel gave it to me and the link to your answer) for me it is a duplicate question as suggested and I marked so. Sorry for my poor english. But it is not a duplicate, and because of an english suptility I can express, please fill free to clarify my question. Thank you for your concern and for your answer. Btw I use it in tikzpicture not with items. – Tarass Oct 13 '14 at 05:23