This is what I came up with so far:
I'm trying to achieve the right column to be vertically centered over the parent rows like the cell containing "Puffer" so that the previous columns aren't affected. I don't really want a fixed length here, because I just want basically a list of items.
Unfortunately it's always aligned at the top and seems to be rendered just inside the first of the multirow cells which crashes the whole design.
I've already tried it with stack and itemize, but I seem to miss something more basic here.
Hopefully someone can help me out here because I can't find a proper or understandable solution anywhere.
\documentclass[12pt,a4paper]{article}
\usepackage[a4paper,lmargin={2.5cm},rmargin={2.5cm},tmargin={3cm},bmargin={2.5cm}]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[ngerman, english]{babel}
\usepackage{makecell}
\usepackage[table]{xcolor}
\usepackage{multirow}
\newcommand{\arr}{$\rightarrow$ }
\usepackage{array}
\usepackage{enumitem}
\usepackage{latexsym}
\usepackage{caption}
\begin{document}
\begin{table}[ht]
\centering
%\rowcolors{2}{gray!25}{white}
\begin{tabular}[c]{|c|c|c|}\cline{3-3}
\multicolumn{2}{c|}{\multirow{1}{*}{}}&\multicolumn{1}{c|}{Inhalt}\\\cline{3-3}
\hline
\multicolumn{1}{|c|}{
\multirow{4}{*}{12/2020}}&
1. Woche&
\multicolumn{1}{|c|}{\multirow{3}{*}{
\makecell{\arr Test \\ \arr Test}
}}\\\cline{2-2}&
2. Woche&
\multicolumn{1}{|c|}{}\\\cline{2-3}&
3. Woche&
\multicolumn{1}{|c|}{\multirow{2}{*}{\arr Puffer}}\\\cline{2-2}&
4. Woche&
\multicolumn{1}{|c|}{}\\
\hline
\end{tabular}
\end{table}
\end{document}
Update:
I've added @Bernard's simplified version and added more text to it to see if it still gets aligned correctly. The code:
\begin{table}\setlength{\extrarowheight}{2pt}
\begin{tabular}{|c|c|l|}\cline{3-3}
\multicolumn{2}{c|}{\multirow{1}{*}{}}&\multicolumn{1}{c|}{Inhalt}\\\cline{3-3}
\hline
\multirow{4}{*}{12/2020} &
1. Woche &
\multirowcell{2}{\arr Prepare samples \\[2pt] \arr Implement Feedback}
\\\cline{2-2}&
2. Woche &
\\ \cline{2-3}&
3. Woche & \multirow{2}{*}{\arr Puffer} \\\cline{2-2}&
4. Woche & \\
\hline
\end{tabular}
\end{table}
\documentclass
,\begin{documnet}
and so on are missing. It really makes it easier to help you if we can simply copy and paste a running code to reproduce your problems. – Steradiant Jun 23 '20 at 11:25