Heading subsubsection without number

Question

I'm writing my thesis and, if I insert a subsubsection, it appears without number. Therefore, if I refer to it, LaTeX works properly, that is it refers with its true number (i.e. 1.1.1). Why? How can I fix it?

This is a part of my preamble:.

\documentclass[12pt,oneside,a4paper]{scrbook}
\usepackage{graphicx}
\usepackage[font=footnotesize,labelfont={bf,bf}]{caption}
\usepackage{amssymb}
\usepackage{amstext, empheq, amsthm}
\usepackage{mathtools}
\usepackage{url}
\usepackage{setspace}
\usepackage{subfig}
\usepackage{fancyhdr}
\usepackage{hyperref}
\usepackage{float}
\usepackage{lmodern}
\usepackage{minitoc}
\usepackage{natbib}

\pagestyle{fancy} 
\fancyhf{} 
\lhead{\nouppercase{\leftmark}}
\fancyhead[R]{\thepage}

\usepackage{pdflscape}
\usepackage{afterpage}