\documentclass[11pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[margin=1in]{geometry}
\usepackage[most]{tcolorbox}
\begin{document}
\begin{tcbposter}[
	poster = {showframe,height=2.5cm,spacing=2mm,rows=2},
  boxes  = {beamer,colframe=blue!50!black,colback=blue!50,colupper=yellow!50},
]
	\posterbox{row=1,column=1}{First box} \posterbox{row=1,column=2,rowspan=2}{%
		\addtolength{\leftmargini}{2cm}
		\begin{itemize}
		\item \small a
		\item \small b
		\end{itemize}
	}
	\posterbox[natural height]{row=1,column=3}{Third box}
\end{tcbposter}
\end{document}