How can I resize the images?

I know two options to easily create a pleasant layout.

(1) using the subfigure environment, or

(2) relying on tabular, which frankly seems much simpler.

I added the length figwidth which allows you to change the width of the figures and see how they fit on the page.

The prototype assumed that the leftmost figure is a square 2/3 the width of the other figures.

documentclass[12pt,a4paper]{article}

usepackage{graphicx}
usepackage{subcaption}
usepackage{kantlipsum} % dummy text    
usepackage[left=1.2in, right=1.2in, top=1in, bottom=1in]{geometry}

newlength{figwidth}
setlength{figwidth}{1.0in}

begin{document}
            
1. kant[1]
    
begin{figure*}[h]
    centering  
    begin{subfigure}[t]{figwidth}
        centering
        includegraphics[height=.66figwidth,width=figwidth]{example-image-a}
    end{subfigure}%
    hspace*{10pt}
    begin{subfigure}[t]{figwidth}
        centering
        includegraphics[height=.66figwidth,width=figwidth]{example-image-a}
    end{subfigure}%
    hspace*{10pt}
    begin{subfigure}[t]{0.66figwidth}
        centering
        includegraphics[height=.66figwidth,width=.66figwidth]{example-image-a}
    end{subfigure}%

    vspace*{10pt}
    begin{subfigure}[t]{figwidth}
        centering
        includegraphics[height=.66figwidth,width=figwidth]{example-image-b}
    end{subfigure}% 
    hspace*{10pt}
    begin{subfigure}[t]{figwidth}
        centering
        includegraphics[height=.66figwidth,width=figwidth]{example-image-b}
    end{subfigure}%
    hspace*{10pt}
    begin{subfigure}[t]{0.66figwidth}
        centering
        includegraphics[height=.66figwidth,width=0.66figwidth]{example-image-b}
    end{subfigure}%
    caption{Images with subfigure.}
end{figure*}

    3. kant[3]
newpage
    
begin{figure*}[h]
    centering
    begin{tabular}{ccc}
        includegraphics[height=.66figwidth,width=figwidth]{example-image-a}&
        includegraphics[height=.66figwidth,width=figwidth]{example-image-a}&
        includegraphics[height=.66figwidth,width=0.66figwidth]{example-image-a}  [6pt] %vertical space between the rows
        includegraphics[height=.66figwidth,width=figwidth]{example-image-b}&
        includegraphics[height=.66figwidth,width=figwidth]{example-image-b}&
        includegraphics[height=.66figwidth,width=0.66figwidth]{example-image-b}  
    end{tabular}
        caption{Images with tabular.}
end{figure*}

2. kant[2]
    
end{document}

This is what you get changing figwidth from 1.8in to 1in. See how with the tabular the vertical and horizontal separation does not need to be re-equalized.

You don't have to place each image inside a subfigure environment if you don't plan on indexing them. As such, just insert the images as-is with a space (or some other width) between them:

documentclass{article}

usepackage{graphicx,lipsum}

begin{document}

lipsum[1]

begin{figure}[ht]
  centering
  includegraphics[height=1in]{example-image-a}space
    includegraphics[height=1in]{example-image-b}space
    includegraphics[height=1in]{example-image-1x1}
  
  includegraphics[height=1in]{example-image-c}space
    includegraphics[height=1in]{example-image}space
    includegraphics[height=1in]{example-image-1x1}
    
  caption{A figure caption}
end{figure}

lipsum[2]

end{document}

From here it should be straight-forward to adjust the height to something suitable for your needs.

---

If you want the images to span the maximum possible width, then set them using some comfortable measure between them, then resizebox the entire row to fit linewidth:

documentclass{article}

usepackage{graphicx,lipsum}

begin{document}

lipsum[1]

begin{figure}[ht]
  centering
  resizebox{linewidth}{!}{%
    includegraphics[height=1in]{example-image-a}space
      includegraphics[height=1in]{example-image-b}space
      includegraphics[height=1in]{example-image-1x1}}
  
  resizebox{linewidth}{!}{%
    includegraphics[height=1in]{example-image-c}space
      includegraphics[height=1in]{example-image}space
      includegraphics[height=1in]{example-image-1x1}}
    
  caption{A figure caption}
end{figure}

lipsum[2]

end{document}