With the enumitem package you can define your own itemize like enviroment that you can use for lists inside table cells. tabularx additionally helps automatically make the table span the textwidth:
\documentclass[a4paper]{article}
\usepackage{multirow}
\usepackage{array}
\usepackage{tabularx}
\usepackage{enumitem}
\newlist{tabitemize}{itemize}{1}
\setlist[tabitemize]{nosep,
topsep= 0pt,
partopsep=0pt,
leftmargin= *,
label=\textbullet,
before=\vspace{0.3\baselineskip},
after=\vspace{-\baselineskip}
}
\renewcommand\tabularxcolumn[1]{m{#1}}
\newcolumntype{M}[1]{>{\centering\arraybackslash}m{#1}}
\begin{document}
\begin{tabularx}{\textwidth}{| M{1.5cm} | M{2.25cm} | X |}
\hline
\multirow{3}{1.5cm}{\centering Deep Learning}
& Medical Image Analysis
& \begin{tabitemize}
\item Built a deep learning model using Mask R-CNN for instance segmentation of nuclei images
\item another long line of text
\end{tabitemize}\\
\cline{2-3}
& Image \newline Classification
& \begin{tabitemize}
\item first long line of text
\item another long line of text
\end{tabitemize}\\
\hline
\multirow{3}{1.5cm}{\centering Time Series}
& Walmart Sales Forecasting
& \begin{tabitemize}
\item long text
\end{tabitemize} \\
\cline{2-3}
& Rossman Sales Forecast
& \begin{tabitemize}
\item long text
\end{tabitemize} \\
\hline
\end{tabularx}
\end{document}

Personally, I would recommend a different design:

Here, I have removed all vertical lines and replaced the horizontal rules by rules from the booktabs package. Additionally, I have aligned all entries of the table cells to the top and added some vertical white space to guide the eye.
\documentclass[a4paper]{article}
\usepackage{tabularx}
\usepackage{enumitem}
\newlist{tabitemize}{itemize}{1}
\setlist[tabitemize]{nosep,
topsep= 0pt,
partopsep=0pt,
leftmargin= *,
label=\textbullet,
before=\vspace{-0.6\baselineskip},
after=\vspace{-\baselineskip}
}
\usepackage{booktabs}
\begin{document}
\begin{tabularx}{\textwidth}{ p{1.5cm} p{2.25cm} X }
\toprule
Deep \newline Learning
& Medical Image Analysis
& \begin{tabitemize}
\item Built a deep learning model using Mask R-CNN for instance segmentation of nuclei images
\item another long line of text
\end{tabitemize}\\
\addlinespace[2ex]
& Image \newline Classification
& \begin{tabitemize}
\item first long line of text
\item another long line of text
\end{tabitemize}\\
\midrule
Time \newline Series
& Walmart Sales Forecasting
& \begin{tabitemize}
\item long text
\end{tabitemize} \\
\addlinespace[2ex]
& Rossman Sales Forecast
& \begin{tabitemize}
\item long text
\end{tabitemize} \\
\bottomrule
\end{tabularx}
\end{document}