documentclass[12 pt,a4paper, oneside, openany, notitlepage]{article}
usepackage{subcaption}
usepackage{mathtools}
newcommand{mathsym}[1]{{}}
newcommand{unicode}[1]{{}}
newcounter{mathematicapage}
usepackage{amsthm}
usepackage{amsfonts}
usepackage{multicol}
usepackage[inner=1in,outer=1in,bottom=1in,top=1in]{geometry}
usepackage{setspace}
onehalfspacing
usepackage{changepage}
usepackage{caption}



begin{document}



begin{figure}
 begin{adjustwidth}{-2cm}{}
centering
    begin{subfigure}[b]{0.37textwidth}
            includegraphics[width=textwidth]{case1_1.jpg}
    end{subfigure}
    begin{subfigure}[b]{0.37textwidth}
            centering
            includegraphics[width=textwidth]{case1_2.jpg}
    end{subfigure}
   begin{subfigure}[b]{0.37textwidth}
            centering
            includegraphics[width=textwidth]{case1_3.jpg}
    end{subfigure}

    begin{subfigure}[b]{0.37textwidth}
            includegraphics[width=textwidth]{case1_4.jpg}
    end{subfigure}
    begin{subfigure}[b]{0.37textwidth}
            centering
            includegraphics[width=textwidth]{case1_5.jpg}
    end{subfigure}
   begin{subfigure}[b]{0.37textwidth}
            centering
            includegraphics[width=textwidth]{case1_6.jpg}
    end{subfigure}
caption{Blah. }
label{case1_fig}
end{adjustwidth}
end{figure}


end{document}

documentclass[12 pt,a4paper, oneside, openany, notitlepage]{article}
usepackage{subcaption}
usepackage{mathtools}
newcommand{mathsym}[1]{{}}
newcommand{unicode}[1]{{}}
newcounter{mathematicapage}
usepackage{amsthm}
usepackage{amsfonts}
usepackage{multicol}
usepackage[inner=1in,outer=1in,bottom=1in,top=1in]{geometry}
usepackage{setspace}
onehalfspacing
usepackage{changepage}
usepackage{caption}

begin{document}

begin{figure}
    centering
    begin{subfigure}[b]{textwidth/3}
            includegraphics[width=textwidth]{example-image-a}
    end{subfigure}%
    begin{subfigure}[b]{textwidth/3}
            includegraphics[width=textwidth]{example-image-b}
    end{subfigure}%
    begin{subfigure}[b]{textwidth/3}
            includegraphics[width=textwidth]{example-image-c}
    end{subfigure}

    begin{subfigure}[b]{textwidth/3}
            includegraphics[width=textwidth]{example-image-c}
    end{subfigure}%
    begin{subfigure}[b]{textwidth/3}
            includegraphics[width=textwidth]{example-image-b}
    end{subfigure}%
    begin{subfigure}[b]{textwidth/3}
            includegraphics[width=textwidth]{example-image-a}
    end{subfigure}
    caption{Blah. }
    label{case1_fig}
end{figure}

end{document}

documentclass{article}
usepackage[margin=1in]{geometry}
usepackage{setspace}
onehalfspacing
usepackage{graphicx}
usepackage{tabularx}
usepackage{caption}

begin{document}
    begin{figure}
    centering
    setlengthtabcolsep{0pt}
    begin{tabularx}{textwidth}{XXX}
includegraphics[width=linewidth]{example-image-a}
    &   includegraphics[width=linewidth]{example-image-b}
        &   includegraphics[width=linewidth]{example-image-c} \[-3pt]
includegraphics[width=linewidth]{example-image-c}
    &   includegraphics[width=linewidth]{example-image-b}
        &   includegraphics[width=linewidth]{example-image-a}
    end{tabularx}
caption{Blah. }
label{case1_fig}
    end{figure}
end{document}

documentclass{article}
usepackage{graphicx}
begin{document}
begin{figure}
includegraphics[width=.333linewidth]{example-image}%
includegraphics[width=.333linewidth]{example-image}%
includegraphics[width=.333linewidth]{example-image}\
includegraphics[width=.333linewidth]{example-image}%
includegraphics[width=.333linewidth]{example-image}%
includegraphics[width=.333linewidth]{example-image}%
caption{Blah. }
end{figure}
end{document}