Autre possibilité: plusieurs boucles de `\multiframe`:
\documentclass[french,11pt,a4paper]{article}
\usepackage{tikz}
\usepackage{animate}
\usepackage{siunitx}
\begin{document}
\begin{center}
\begin{animateinline}[controls=all]{25}
\multiframe{50}{rx=0+0.02}{%
\begin{tikzpicture}
\draw[thin,lightgray] (0,0) grid (7,4) ;
\draw[fill=blue] (1+\rx,1) circle (0.2cm);
\draw[fill=red] (2,2) circle (0.2cm);
\draw[fill=green] (3,3) circle (0.2cm);
\end{tikzpicture}
}
\newframe
\newframe
\multiframe{50}{rx=1+0.02}{%
\begin{tikzpicture}
\draw[thin,lightgray] (0,0) grid (7,4) ;
\draw[fill=blue] (2,1) circle (0.2cm);
\draw[fill=red] (1+\rx,2) circle (0.2cm);
\draw[fill=green] (3,3) circle (0.2cm);
\end{tikzpicture}
}
\newframe
\multiframe{51}{rx=2+0.02}{%
\begin{tikzpicture}
\draw[thin,lightgray] (0,0) grid (7,4) ;
\draw[fill=blue] (2,1) circle (0.2cm);
\draw[fill=red] (3,2) circle (0.2cm);
\draw[fill=green] (1+\rx,3) circle (0.2cm);
\end{tikzpicture}
}
\end{animateinline}
\end{center}
\end{document}
