\documentclass[letterpaper,11pt]{article}
\vspace{-3pt}\item
\begin{tabular*}{\textwidth}[t]{l@{\extracolsep{\fill}}r}
\textbf{
London School of Economics and Political Science (LSE)
} &
London, United Kingdom \\
\textit{\small
Data Science and Big Data Analytics (Summer School); Grade: A
} & \textit{\small
July 2018 -- August 2018} \\
\end{tabular*}\vspace{-5pt}
\end{document}
How it looks like (see the left misalignment of the two rows)

%after\textbf{, otherwise a space will be inserted there. It should be\textbf{%. Or, since a table cell limits the font change to the current cell, you can simply replace\textbf{text}by\bfseries text. – Phelype Oleinik Nov 02 '19 at 02:15\textit{\smalllines because the\smallabsorbs the line end/space that follows it. – moewe Nov 02 '19 at 07:20