With tcolorbox
\documentclass{beamer}
\usepackage[most]{tcolorbox}
\newtcolorbox{myblock}[1]{beamer, width=.475\linewidth,title=#1,on line}
\begin{document}
\begin{frame}
\begin{myblock}{First block}
Some text
\end{myblock}\hfill
\begin{myblock}{Second block}
Some text
\end{myblock}
\bigskip
{\par\centering
\begin{myblock}{Third block}
Some text
\end{myblock}
\par}
\end{frame}
\end{document}

With minipages
and beamer
blocks.
\documentclass{beamer}
\usetheme{Boadilla}
\begin{document}
\begin{frame}
\begin{minipage}[t]{.45\linewidth}
\begin{block}{First block}
Some text
\end{block}
\end{minipage}\hfill
\begin{minipage}[t]{.45\linewidth}
\begin{block}{Second block}
Some text
\end{block}
\end{minipage}
\bigskip
{\par\centering
\begin{minipage}[t]{.45\linewidth}
\begin{block}{Third block}
Some text
\end{block}
\end{minipage}
\par}
\end{frame}
\end{document}

A similar result can be obtained with beamer's beamerboxesrounded
boxes and columns
.
\documentclass{beamer}
\usetheme{Boadilla}
\begin{document}
\begin{frame}
\begin{columns}[onlytextwidth]
\column{.45\linewidth}
\begin{beamerboxesrounded}[width=\linewidth,shadow]{First block}
Some text
\end{beamerboxesrounded}%
\column{.45\linewidth}
\begin{beamerboxesrounded}[width=\linewidth,shadow]{Second block}
Some text
\end{beamerboxesrounded}
\end{columns}
\bigskip
\begin{columns}[onlytextwidth]
\column{.275\linewidth}
\column{.45\linewidth}
\begin{beamerboxesrounded}[width=\linewidth,shadow]{Third block}
Some text
\end{beamerboxesrounded}
\column{.275\linewidth}
\end{columns}
\end{frame}
\end{document}