\begin{tikzpicture}
\begin{umlseqdiag}
\umlobject[class=A]{a}
\umlobject[class=B]{b}
\umlobject[class=C]{c}
\begin{umlcall}{a}{b}
\begin{umlcall}{b}{c}
\end{umlcall}
\end{umlcall}
\end{umlseqdiag}
\end{tikzpicture}