Table multirow vertical center alignment not working [duplicate]

At Using multirow package you should consider all text lines in adjacent columns. In your case 10 of them:

\documentclass{book}
\usepackage{multirow}
\begin{document}
\noindent%
\begin{tabular}{p{100pt}p{100pt}p{100pt}}
    \hline
\multirow{10}{=}{realized two line text two line text two line text}
        &   realized two line text two line text two line text
            &   realized two line text two line text two line text\\
        &   realized two line text two line text two line text
            &   realized two line text two line text two line text\\
        & 1 & 2\\
        & 1 & 2\\
        & 1 & 2\\
        & 1 & 2\\
    \hline
\end{tabular}
\end{document} 

Or by using tabularray package:

\documentclass{book}
\usepackage{tabularray}
\begin{document}
\noindent%
\begin{tblr}{colspec = { *{3}{Q[l, wd=100pt]}},
             cell{1}{1} = {r=6}{}   % observe diference in number of rows
             }
    \hline
realized two line text two line text two line text
    &   realized two line text two line text two line text
        &   realized two line text two line text two line text\\
    &   realized two line text two line text two line text
        &   realized two line text two line text two line text\\
    & 1 & 2\\
    & 1 & 2\\
    & 1 & 2\\
    & 1 & 2\\
    \hline
\end{tblr}
\end{document} 

Result is the same as before.

I'd like to suggest using a tabularx environment with three X-type columns. This will assure that the table's width doesn't exceed \textwidth.

\documentclass{book}
\usepackage{multirow,tabularx}
\begin{document}
\noindent
\begin{tabularx}{\textwidth}{XXX}
\hline
\multirow{11}{\hsize}{%
realized two line text two line text two line text}
&realized two line text two line text two line text
&realized two line text two line text two line text\\
&realized two line text two line text two line text
&realized two line text two line text two line text\\
&1&2\\
&1&2\\
&1&2\\
&1&2\\
&1&2\\
\hline
\end{tabularx}
\end{document}

This puts one tabular inside another.

\documentclass{book}
%\usepackage{multirow} % no longer needed
\usepackage{array} % for 'm' column type
\begin{document}
\noindent
\begin{tabular}{ m{100pt} c }
\hline
realized two line text two line text two line text &
  \begin{tabular}{@{} p{100pt} p{100pt} @{}}
  realized two line text two line text two line text&
  realized two line text two line text two line text\\
  realized two line text two line text two line text&
  realized two line text two line text two line text\\
  1&2\\
  1&2\\
  1&2\\
  1&2
  \end{tabular}\\
\hline
\end{tabular}
\end{document}