How to create subfloat figures (two in first row and one below)?

Here's a possible solution using the subfigure environment from the subcaption package:

\documentclass{article}
\usepackage[demo]{graphicx}
\usepackage{caption}
\usepackage{subcaption}

\begin{document}

\begin{figure}
\begin{subfigure}{.5\linewidth}
\centering
\includegraphics{image1}
\caption{}
\label{fig:sub1}
\end{subfigure}%
\begin{subfigure}{.5\linewidth}
\centering
\includegraphics{image2}
\caption{}
\label{fig:sub2}
\end{subfigure}\\[1ex]
\begin{subfigure}{\linewidth}
\centering
\includegraphics{image3}
\caption{}
\label{fig:sub3}
\end{subfigure}
\caption{Three subfigures}
\label{fig:test}
\end{figure}

As we can see in Figures~\ref{fig:sub1}, \ref{fig:sub2}, and~\ref{fig:sub3}...

\end{document}

The demo option for graphicx was only used to replace actual figures with black rectangles. Do not use that option in your actual document,

If I understand what you would like, then one method would be to use a couple of minipages and the subfig package. Spacing etc. could be adjusted to your needs:

\documentclass{article}
\usepackage{mwe}
\usepackage{subfig}
\begin{document}

\begin{figure}

\begin{minipage}{.5\linewidth}
\centering
\subfloat[]{\label{main:a}\includegraphics[scale=.5]{example-image-a}}
\end{minipage}%
\begin{minipage}{.5\linewidth}
\centering
\subfloat[]{\label{main:b}\includegraphics[scale=.5]{example-image-b}}
\end{minipage}\par\medskip
\centering
\subfloat[]{\label{main:c}\includegraphics[scale=.5]{example-image-c}}

\caption{my fig}
\label{fig:main}
\end{figure}

\end{document}

Using a linebreak (similar to Gonzalo's answer) the subfig package does the trick even without minipages.

\documentclass{article}
\usepackage{subfig}
\usepackage[demo]{graphicx}

\begin{figure}[ht]%
 \centering
 \subfloat[]{\includegraphics{UEB01-repograph1.pdf}\label{fig:a}}%
 \subfloat[]{\includegraphics{UEB01-repograph2.pdf}\label{fig:b}}\\
 \subfloat[]{\includegraphics{UEB01-repograph3.pdf}\label{fig:c}}%
 \caption{Some caption}%
 \label{some-label}%
\end{figure}

\end{document}

There might be drawbacks I don't know about in more complex scenarios.