I have a circle in the picture... How to add dimension of radius... form center of the circle to any point on the circle? Rotation is not giving what i want...

```
\documentclass[10pt]{article}
\usepackage{tikz}

\begin{document}

 \begin{tikzpicture}[scale=1.0]

\draw [step=1.0,thin,gray!40] (0,-3) grid (6,3);

\fill[blue] (3,0) circle (2pt) node [black,,below left] {$C$};
%
\draw[thick] (3,0) circle(3);

\draw[|->|] (3,0) -- (6,0)  node [midway,fill=white] {3$a$};
\draw[|->|,rotate=45] (3,0) -- (6,0)  node [midway,fill=white] {3$a$};

\end{tikzpicture}

\end{document}

```