documentclass[journal]{IEEETran}
ifCLASSINFOpdf
  usepackage[pdftex]{graphicx}
  graphicspath{{pdf/}{jpeg/}{png/}}
  DeclareGraphicsExtensions{.pdf,.jpeg,.png}
else
  usepackage[dvips]{graphicx}
  graphicspath{{../eps/}}
  DeclareGraphicsExtensions{.eps}
fi
ifCLASSOPTIONcompsoc
  usepackage[caption=false,font=normalsize,labelfont=sf,textfont=sf]{subfig}
else
  usepackage[caption=false,font=footnotesize]{subfig}
fi

begin{document}
begin{figure}
    centering
    subfigure[]{includegraphics[width=0.8linewidth]{fig1-a.png}}
    vfill
    subfigure[]{includegraphics[width=0.8linewidth]{fig1-b.pdf}}
    vfill
    subfigure[]{includegraphics[width=0.8linewidth]{fig1-c.png}}
    caption{...}
    label{fig2}
end{figure}
end{document}