usepackage{array}
usepackage{tabularx}
usepackage{subcaption,siunitx,booktabs}
usepackage[margin=1in]{geometry}
usepackage{multirow, makecell}
usepackage{array}
setlengthextrarowheight{2pt}

begin{table*}
begin{subtable}{1textwidth}

centering
begin{tabular}{|l|l|l|l|}
    hline
    multirow{2}{*}{Dataset} &
      multicolumn{2}{c|}{A} & \
    & O.B.R & A.R
    & {Dataset2}\
    hline
    D1 & 2.1% & 2.1% & D11 \
    hline
    D2 & 11.6% & 11.6% & D21\
    hline
    D3 & 5.5% & 5.5% & D31\
    hline
end{tabular}
   caption{First subtable1}label{tab:sub_first1}
end{subtable}
caption{Three simple tables} label{tab:three_tables}
end{table*}