\begin{tikzpicture}
\umlemptyclass{A}
\umlemptyclass[x=6]{B}
\umlemptyclass[x=3, y=3]{C}
\umlNarynode[x=3, name=naryassoc, below]{label}
\umlassoc[attr1=a|*]{A}{naryassoc}
\umlassoc[mult1=0..1]{B}{naryassoc}
\umlassoc[mult=*]{naryassoc}{C}
\end{tikzpicture}