Table with all numbers in millions

Edit: I just saw that you are from Germany. In this case a "," as group seperator is not such a good idea as it might be confused with decimal seperators. I'll use a simple small space then.

\documentclass{article}

\usepackage{siunitx}
\usepackage{booktabs}

\begin{document}

\begin{tabular}{
    l
    *{3}{S[fixed-exponent = 3,
           round-precision = 3,
           round-mode=figures,
           group-minimum-digits = 4,
           group-separator = {\,},
           table-omit-exponent, 
           table-format = 5.0]}
    r}
    \toprule
    Category & Col A / \num{e3} & Col B / \num{e3} &  Col C / \num{e3} & \dots\\ 
    \midrule
    Line 1 & 18855000 & 13870000 & 1235000 & \dots\\
    \addlinespace
    Line 2\\
    \quad Item 2--1 & 10280000 & 7519000 & 650000 & \dots\\
    \quad Item 2--2 & 8575000 & 6351000 & 585000 & \dots\\
    \dots\\
    \bottomrule
\end{tabular}

\end{document}

Or maybe like this?

\documentclass{article}
\usepackage[%
    %,locale=DE % if you write in Germany, switch that on for comma decimal seperators
]{siunitx}
%\sisetup{input-decimal-markers = ., input-ignore = {,}} % if your input looks like in your MWE
\usepackage{booktabs}

\begin{document}

\begin{table}
\caption{Amounts in millions}
\centering
\begin{tabular}{%
    l
    S[fixed-exponent = 6, table-omit-exponent,table-format = 2.3, table-auto-round]
    S[fixed-exponent = 6, table-omit-exponent,table-format = 2.3, table-auto-round]
    S[fixed-exponent = 6, table-omit-exponent,table-format = 1.3, table-auto-round]
    r
    }
    \toprule
    Category & {Col A} & {Col B} &  {Col C} & \dots\\ 
    \midrule
    Line 1 & 18855000 & 13870000 & 1235000 & \dots\\
    \addlinespace
    Line 2\\
    \quad Item 2--1 & 10280000 & 7519000 & 650000 & \dots\\
    \quad Item 2--2 & 8575000 & 6351000 & 585000 & \dots\\
    \dots\\
    \bottomrule
\end{tabular}
\end{table}

\end{document}

Edit

DIN 55301 - "Gestaltung statistischer Tabellen" recommends to use the caption (title) for the appearance of "millions". For single columns, they use e.g. "Col A in millions" or for just a unit "Mill. [Col A]" or "M[Col A]"*. The example for our case of all occurring values says: "Table 1: Amounts in 1000000"

*[X] means: unit of X