How to align multiple subfigures in a grid?

You are already using the subcaption package, and it is very easy with it.

Here's the code you need

\documentclass{article}
\usepackage{graphicx}
\usepackage{subcaption}

\begin{document}

\begin{figure}
\centering
\begin{subfigure}[b]{.45\linewidth}
\includegraphics[width=\linewidth]{example-image-a}
\caption{A mouse}\label{fig:mouse}
\end{subfigure}

\begin{subfigure}[b]{.45\linewidth}
\includegraphics[width=\linewidth]{example-image-b}
\caption{A gull}\label{fig:gull}
\end{subfigure}
\begin{subfigure}[b]{.45\linewidth}
\includegraphics[width=\linewidth]{example-image-c}
\caption{A tiger}\label{fig:tiger}
\end{subfigure}
\caption{Picture of animals}
\label{fig:animals}
\end{figure}

\end{document}

Output:

If you need to have the subcaption numbers as in your picture, you have to adjust the counter subfigure with \setcounter{subfigure}{<num>} inside the subfigures

\documentclass{article}
\usepackage{graphicx}
\usepackage{subcaption}

\begin{document}

\begin{figure}
\centering
\begin{subfigure}[b]{.45\linewidth}
\includegraphics[width=\linewidth]{example-image-c}
\setcounter{subfigure}{2}%
\caption{A mouse}\label{fig:mouse}
\end{subfigure}

\begin{subfigure}[b]{.45\linewidth}
\includegraphics[width=\linewidth]{example-image-a}
\setcounter{subfigure}{0}%
\caption{A gull}\label{fig:gull}
\end{subfigure}
\begin{subfigure}[b]{.45\linewidth}
\includegraphics[width=\linewidth]{example-image-b}
\caption{A tiger}\label{fig:tiger}
\end{subfigure}
\caption{Picture of animals}
\label{fig:animals}
\end{figure}

\end{document} 

Output:

It's relatively easy with the floatrow package, and its subfloatrowenvironment:

\documentclass{article}
\usepackage{subcaption}
\usepackage{floatrow}
\usepackage{graphicx}
\usepackage[showframe]{geometry}

\begin{document}

\begin{figure}
\floatsetup{capposition = below, floatrowsep =qquad,}
\centering
\begin{subfloatrow}
\centering
  \ffigbox[\textwidth]{\caption{The Death of Procris}}{%
  \includegraphics[width=0.8\textwidth]{Piero_di_Cosimo_1}
  }
\end{subfloatrow}
\vskip1cm
\begin{subfloatrow}
    \ffigbox[0.5\textwidth]{\caption{Portrait of Simonetta Vespucci}}{%
  \includegraphics[width=0.4\textwidth]{Piero_di_Cosimo_3}%
  }
    \ffigbox[0.5\textwidth]{\caption{Portrait of Simonetta Vespucci}}{%
  \includegraphics[width=0.4\textwidth]{Piero_di_Cosimo_3}%
  }
\end{subfloatrow}
\caption{Three paintings by Piero di Cosimo}
\label{PC}
\end{figure}

\end{document} 

Within a figure environment you can start a tabular environment to have more control over the distribution of space. Fill the tabs with subfigures. You can even use the multirow package for even further tweaking and control.

\documentclass{article}

\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{multirow}

\begin{document}

\begin{figure}[h]
  \centering
  \begin{tabular}[c]{ccccc}
    \multirow{2}{*}[14pt]{
    \begin{subfigure}{0.45\textwidth}
      \includegraphics[height=2cm, width=\textwidth]{example-image-b}
      \caption{CEO fixed / total}
      \label{fig:ceoa}
    \end{subfigure}
}&
   \begin{subfigure}[c]{0.11\textwidth}
      \includegraphics[height=1cm, width=\textwidth]{example-image-a}
      \label{fig:ceob}
    \end{subfigure}&
    \begin{subfigure}[c]{0.11\textwidth}
      \includegraphics[height=1cm, width=\textwidth]{example-image-a}
      \label{fig:ceoc}
    \end{subfigure}&
    \begin{subfigure}[c]{0.11\textwidth}
      \includegraphics[height=1cm, width=\textwidth]{example-image-a}
      \label{fig:ceoe}
    \end{subfigure}&
    \begin{subfigure}[c]{0.11\textwidth}
      \includegraphics[height=1cm, width=\textwidth]{example-image-a}
      \label{fig:cdfeod}
    \end{subfigure}\\
    &
        \begin{subfigure}[c]{0.11\textwidth}
      \includegraphics[height=1cm, width=\textwidth]{example-image-a}
      \label{fig:caeob}
    \end{subfigure}&
    \begin{subfigure}[c]{0.11\textwidth}
      \includegraphics[height=1cm, width=\textwidth]{example-image-a}
      \label{fig:ceoac}
    \end{subfigure}&
    \begin{subfigure}[c]{0.11\textwidth}
      \includegraphics[height=1cm, width=\textwidth]{example-image-a}
      \label{fig:ceose}
    \end{subfigure}&
    \begin{subfigure}[c]{0.11\textwidth}
      \includegraphics[height=1cm, width=\textwidth]{example-image-a}
      \label{fig:ceod}
    \end{subfigure}\\
  \end{tabular}    
  \caption{Compensation measures}
  \label{fig:ceo}
\end{figure}

\end{document}