Das kannst du mit den KOMA-eigenen Befehlen machen:
machen und nicht mit `titlesec`:
\documentclass[12pt,twoside,BCOR=12mm,DIV=calc]{scrbook}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage{libertine}
\RedeclareSectionCommand[
style =section,
indent=0pt,
font =\Huge\bfseries\scshape,
beforeskip=-6ex plus -0.5ex minus -0.2ex,
afterskip = 2ex plus 0.2ex]{chapter}
\RedeclareSectionCommand[
style =section,
indent=0pt,
font =\Large\bfseries,
beforeskip=-6ex plus -0.5ex minus -0.2ex,
afterskip = 2ex plus 0.2ex]{section}
\RedeclareSectionCommand[
style =section,
indent=0pt,
font =\large\bfseries,
beforeskip=-6ex plus -0.5ex minus -0.2ex,
afterskip = 2ex plus 0.2ex]{subsection}
\begin{document}
\chapter{Einleitung}
\noindent Lorem ipsum dolor sit amet, consetetur sadipscing elitr,
sed diam nonumy eirmod tempor invidunt ut labore et do-
lore magna aliquyam erat, sed diam voluptua. At vero eos
et accusam et justo duo dolores et ea rebum. Stet clita kasd
\section{Unterabschnitt}
At vero
eos et accusam et justo duo dolores et ea rebum. Stet clita
kasd gubergren, no sea takimata sanctus est Lorem ipsum
\subsection{UnterUnterabschnitt}
amet. Lorem ipsum dolor sit amet, consetetur sadipscing
elitr, sed diam nonumy eirmod tempor invidunt ut labore
et dolore magna aliquyam erat, sed diam voluptua. At vero
eos et accusam et justo duo dolores et ea rebum. Stet clita
kasd gubergren, no sea takimata sanctus est Lorem ipsum
\end{document}
