%% LyX 2.3.5.2 created this file.  For more info, see http://www.lyx.org/.
%% Do not edit unless you really know what you are doing.
\documentclass[english]{article}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\setlength{\parskip}{\bigskipamount}
\setlength{\parindent}{0pt}
\usepackage{amsmath}
\usepackage{babel}
 
\usepackage{array} % fittet vertikale Linien
% Höhenkorrektur einzelner Zellen:
\newcommand\hhh{\rule[-1em]{0pt}{2.5em}} 
 
 
% Unnötig =============
%\makeatletter
% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%% LyX specific LaTeX commands.
%%% Because html converters don't know tabularnewline
%\providecommand{\tabularnewline}{\\}
% \makeatother
 
\begin{document}
\textsf{Wie soll es denn aussehen?}
 
\textsf{So?}
\begin{tabular}{|c|l|c|}
\hline
    & A                       & B      \\ \hline
(1) & $\dfrac{3}{4}$\hhh      & 42      \\ \hline
(1) & $\sqrt{a^{2}}$          & $\sum$  \\ \hline
\end{tabular}
 
\bigskip
\textsf{Oder so?}
\begin{tabular}{c|l|c|}
\cline{2-3}   
                          & A                    & B      \\ \hline
\multicolumn{1}{|l|}{(1)} & $\dfrac{3}{4}$\hhh   & 42     \\ \hline
\multicolumn{1}{|l|}{(2)} & $\sqrt{a^{2}}$       & $\sum$ \\ \hline
\end{tabular}
 
%Alt:
%\begin{tabular}{|c|l|c|}
%\hline 
% & (A) & (B)%\tabularnewline
%\\ \hline\hline 
%(1) & $\dfrac{3}{4}$ & 42\tabularnewline \hline
%(2) & $\sqrt{a^{2}}$ & $\sum$\tabularnewline\hline 
%\end{tabular}
\end{document}

\documentclass[]{article}
\usepackage[T1]{fontenc}
%\usepackage[latin9]{inputenc}
\setlength{\parskip}{\bigskipamount}
\setlength{\parindent}{0pt}
\usepackage{amsmath}
\usepackage[english, ngerman]{babel}
 
\usepackage{array} % fittet vertikale Linien
\begin{document}
\section{Neu}
 
\subsection{Höhenkorrektur aller Zellen}
\def\Aa{}
\def\Ab{A}
\def\Ac{B}
\def\Ba{\multicolumn{1}{|l|}{(1)}}
\def\Bb{$\dfrac{3}{4}$}
\def\Bc{42}
\def\Ca{\multicolumn{1}{|l|}{(2)}}
\def\Cb{$\sqrt{a^{2}}$}
\def\Cc{$\sum$}
 
{\renewcommand{\arraystretch}{2.5}% temporäre Höhenkorrektur
\begin{tabular}{c|l|c|}
\cline{2-3}   
\Aa & \Ab   & \Ac   \\ \hline
\Ba & \Bb   & \Bc    \\ \hline
\Ca & \Cb   & \Cc    \\ \hline
\end{tabular}   }
 
\subsection{Nur lokale Höhenkorrektur}
\def\Aa{}
\def\Ab{A}
\def\Ac{B}
\def\Ba{\multicolumn{1}{|l|}{(1)}}
\def\Bb{$\dfrac{3}{4}$\rule[-1em]{0pt}{2.5em}}
\def\Bc{42}
\def\Ca{\multicolumn{1}{|l|}{(2)}}
\def\Cb{$\sqrt{a^{2}}$\rule[-0.5em]{0pt}{2.0em}}
\def\Cc{$\sum$}
 
\begin{tabular}{c | l | c |}
\cline{2-3}   
\Aa & \Ab   & \Ac   \\ \hline
\Ba & \Bb   & \Bc    \\ \hline
\Ca & \Cb   & \Cc    \\ \hline
\end{tabular}    
\end{document}