When nesting frames the inner most frametitle is used. The frametitle for all these is frame3.
\documentclass{book}
\usepackage{mdframed}
\begin{document}
\begin{mdframed}[frametitle={frame1}]
asdf
\begin{mdframed}[frametitle={frame2}]
134
\begin{mdframed}[ frametitle={frame3}]
asdf1324
\end{mdframed}
\end{mdframed}
\end{mdframed}
\end{document}
(mdframed v1.4)

