\documentclass[notes,11pt, aspectratio=169]{beamer}
\usepackage{booktabs}
\usetheme[progressbar=frametitle]{metropolis}
\usepackage{appendixnumberbeamer}
\usepackage[numbers,sort&compress]{natbib}
\bibliographystyle{plainnat}
\usepackage{booktabs}
\usepackage[scale=2]{ccicons}
\newcommand{\diffcol}[1]{\textcolor{blue}{#1}}
\newcommand{\red}[1]{\textcolor{red}{#1}}
\newcommand{\blue}[1]{\textcolor{blue}{#1}}
\newcommand{\green}[1]{\textcolor{teal}{#1}}
\newenvironment{wideitemize}{\itemize\addtolength{\itemsep}{1em}}{\enditemize}
\usepackage{xspace}
\newcommand{\themename}{\textbf{\textsc{metropolis}}\xspace}
\title{Project Title:\\ Subtitle Header}
\author[Short Name (U ABC)]{%
\texorpdfstring{%
\begin{columns}
\column{.3\linewidth}
\centering
Author 1 \\ Institution 1
\column{.3\linewidth}
\centering
Author 2 \\ Institution 2
\column{.3\linewidth}
\centering
Author 3 \\ Institution 3
\end{columns}
}
{Author 1, Author 2, Author 3}
}
\date{} % For some reason I can't get the date to be centered
\begin{document}
\maketitle
\begin{frame}{Table of contents}
\setbeamertemplate{section in toc}[sections numbered]
\tableofcontents[hideallsubsections]
\end{frame}
\section{Section}
\begin{frame}{ List }
\begin{wideitemize}
\item[] Outside item
\bigskip
\begin{wideitemize}
\item Inside item
\end{wideitemize}
\end{wideitemize}
\end{frame}
\end{document}
