The value is stored in \beamer@slideinframe
; you can use this to define either a counter or a user command to have easy access to it (it was not clear to me what you wanted to do with the slide number in your example, so I use here another simple example):
\documentclass{beamer}
\makeatletter
\def\c@slideinframe{\beamer@slideinframe}
\def\beamerslideinframe{\beamer@slideinframe}
\makeatother
\begin{document}
\begin{frame}
\frametitle{Slide \arabic{slideinframe}}
\only<1>{A}
\only<2>{B}
\only<3>{C}
\end{frame}
\end{document}