documentclass{beamer}

% Subsection counter:
newcounter{stappen}
newcommand{snumreset}{setcounter{stappen}{0}}
newcommand{snum}{stepcounter{stappen} Roman{stappen}}

begin{document}
section{Section 1}
subsection{Subsec 1}
snumreset

begin{frame}
  frametitle{subsecnamesnum}
  begin{itemize}
  item<1-> bla
  item<2-> counter should not increase
  item<3-> counter should again not increase
  end{itemize}
end{frame}

begin{frame}
  frametitle{subsecnamesnum}
  Counter should be at two.
end{frame}
end{document}