begin{center}
begin{figure}
  centering
  begin{subfigure}[b]{0.3textwidth}
                includegraphics[natwidth=512px,natheight=511px,width=4cm,height=4cm]{param.png}
                caption{img1}
                label{img1}
        end{subfigure}%
        ~ %add desired spacing between images, e. g. ~, quad, qquad, hfill etc.
          %(or a blank line to force the subfigure onto a new line)
        begin{subfigure}[b]{0.3textwidth}
                includegraphics[natwidth=512px,natheight=511px,width=4cm,height=4cm]{crtpnt.png}
                caption{img2}
                label{fig:img2}
        end{subfigure}
        ~ %add desired spacing between images, e. g. ~, quad, qquad, hfill etc.
          %(or a blank line to force the subfigure onto a new line)
        begin{subfigure}[b]{0.3textwidth}
                includegraphics[natwidth=512px,natheight=511px,width=4cm,height=4cm]{ferro.png}
                caption{img3}
                label{fig:img3}
        end{subfigure}
caption[prueba]{label{fig:images}caption}end{figure}end{center}