Subfigures in triangle with the subfigure package

With subfigure

documentclass{article}
usepackage[margin=0.5in]{geometry}
usepackage[utf8]{inputenc}
usepackage{textcomp}
usepackage{pgfplots}
usepackage{subcaption}

begin{document}


    begin{figure}[ht]

        begin{subfigure}{.5textwidth} % this sets the figure to be max half the width of the page
            centering
            % include first image
            includegraphics[width=.9linewidth]{example-image-b}  % this sets the image to fill 90% of the available space -> 45% of the line width in total. 
            caption{Put your sub-captionB here}
            label{fig:sub-first}
        end{subfigure}
        begin{subfigure}{.5textwidth}
            centering
            % include second image
            includegraphics[width=.9linewidth]{example-image-c}  
            caption{Put your sub-captionC here}
            label{fig:sub-second}
        end{subfigure}

        label{fig:fig}
        begin{subfigure}{textwidth}
            centering
            % include first image
            includegraphics[width=.45linewidth]{example-image-a}   % this width should be half of the width of the other two images
            caption{Put your sub-captionA here}
            label{fig:sub-firstA}
        end{subfigure}
        caption{Put your caption here}
    end{figure}

end{document}

With subfig/subfloat

documentclass{article}
usepackage{subfig}
usepackage[demo]{graphicx}
begin{document}

begin{figure}[ht]%
 centering
 subfloat[]{includegraphics{UEB01-repograph1.pdf}label{fig:a}} quad
 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}

EDIT1

Adding placement with array

documentclass{article}
usepackage{subfig}
usepackage[demo]{graphicx}
begin{document}

begin{figure}[ht]%
 centering
 subfloat[]{includegraphics{UEB01-repograph1.pdf}label{fig:a}} quad
 subfloat[]{includegraphics{UEB01-repograph2.pdf}label{fig:b}}
 subfloat[]{includegraphics{UEB01-repograph3.pdf}label{fig:c}}%
 caption{Some caption}%
 label{some-label}%
end{figure}
begin{figure*}[t!]
$begin{array}{cc}
    includegraphics[width=0.5textwidth]{FIGURE_1.pdf} &
    includegraphics[width=0.5textwidth]{FIGURE_2.pdf}
    textbf{fjhfj}& textbf{fjhfj}
    multicolumn{2}{c}{includegraphics[width=0.5textwidth]{FIGURE_3.pdf}}
    multicolumn{2}{c}{textbf{fjhfj}}
end{array}$
caption[My beautiful figure.]{label{fig:label}My beautiful figure}
end{figure*}
end{document}

With minipage

documentclass{article}
usepackage{graphicx,subcaption}

begin{document}

begin{figure}[htp]
centering

begin{minipage}[b]{0.58textwidth} % almost 60%
begin{subfigure}{0.49textwidth}
includegraphics[width=textwidth]{example-image-a}

caption{Mystery shack}
end{subfigure}%
hfill
begin{subfigure}{0.49textwidth}
includegraphics[width=textwidth]{example-image-b}

caption{Nice landscape}
end{subfigure}


end{minipage} par 
begin{minipage}[b]{0.38textwidth} % almost 40%
includegraphics[width=textwidth]{example-image}

caption{Rainbow gnome}
end{minipage}%
caption{Gravity falls}
end{figure}

end{document}