\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}
\usepackage{tikz}
\usepackage{tkz-fct}
\usetikzlibrary{shapes.misc}
\usetikzlibrary{shapes, calc, decorations}
\usepackage{amsmath,amssymb}

\begin{document}
\begin{preview}
\begin{tikzpicture}[dot/.style={    thick,
    %draw=gray,
    %cross out,
    fill,
    shape=circle,
    inner sep=3pt,
    minimum width=4pt,
    minimum height=4pt}]
\newcommand{\R}{2cm};

\node (a) [dot] at (0,0) {};
\node (b) [dot] at (\R,0) {};
\node (c) [dot] at ({\R*cos(60)},{\R*sin(60)}) {};

%\draw[dashed,thick] (a) circle(\R);
%\draw[dashed,thick] (b) circle(\R);
%\draw[dashed,thick] (c) circle(\R);

\draw[->, ultra thick] (a) -- (b);
\draw[->, ultra thick] (b) -- (c);
\draw[->, ultra thick] (c) -- (a);
\end{tikzpicture}
\end{preview}
\end{document}