“below of=” ? more than one block?

I want to make a block c under two blocks a & b. So far I'm able to do it for only one block

\node [block, below of=a]    (c)    {filter};

• Welcome to TeX.SX! You can have a look at our starter guide to familiarize yourself further with our format. – user11232 Dec 8 '13 at 0:47
• Please add a minimal working example (MWE) that starts with \documentclass{...} and ends with \end{document}. It'll be much easier for us to understand your problem this way. – Herr K. Dec 8 '13 at 0:52

Please always post complete code starting from \documentclass{..} and ending at \end{document}. I don't know how block is defined.

Your question is not clear. Whether a ,b and c are one below the other or a and b side by side with c below them at the center?

Please note that below of=a is deprecated. you should use below = of a instead with \usetikzlibrary{positioning}. Here are the possibilities.

\documentclass[tikz]{standalone}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\node (a) {Coffee};
\node [below = of a] (b) {Sugar};
\node [below = of b] (c) {Filter};
\begin{scope}[xshift=8cm]
\node (a) {Coffee};
\node [below left = of a] (c) {Filter};
\node [above left =  of c] (b) {Sugar};
\end{scope}
\end{tikzpicture}
\end{document}


You can control the distances manually like below left = 1cm and 2cm of a. For details refer pgfmanual.