Here's one for two mediators:
\documentclass[tikz,border=4]{standalone}
\usepackage{siunitx}
\usetikzlibrary{positioning}
\tikzset{mynode/.style={draw,text width=1in,align=center}
}
\begin{document}
\begin{tikzpicture}
\node[mynode] (m1){M1};
\node[mynode, right=3cm of m1] (m2){M2}; % Manual adjustment of sep
\node[mynode,below left=of m1](a){X};
\node[mynode,below right=of m2](b){Y};
\draw[-latex] (a.north) -- node[auto,font=\footnotesize] {A}(m1.west);
\draw[-latex] (m1.east) -- node[auto,font=\footnotesize] {B} (m2.west);
\draw[-latex] (m2.east) -- node[auto,font=\footnotesize] {C} (b.north);
\draw[-latex] (a.east) --
node[below=3mm,font=\footnotesize,align=center] {Direct effect, $b=0.55$,
$p=0.03$ \\ Indirect effect, $b=0.16$, \SI{90}{\percent} CI [ 0.04,0.36]}
(b.west);
\end{tikzpicture}
\end{document}

And another for two parallel single mediators:
\documentclass[tikz,border=4]{standalone}
\usepackage{siunitx}
\usetikzlibrary{positioning}
\tikzset{mynode/.style={draw,text width=1in,align=center}
}
\begin{document}
\begin{tikzpicture}
\node[mynode] (m1){Mediator 1};
\node[mynode,below left=of m1](a) {Variable A};
\node[mynode,below right=of m1](b) {Variable B};
\node[mynode,below = 2.5cm of m1] (m2){Mediator 2};
\draw[-latex] (a.north) -- node[auto,font=\footnotesize] {A} (m1.west);
\draw[-latex] (m1.east) -- node[auto,font=\footnotesize] {B} (b.north);
\draw[-latex] (a.east) --
node[below,font=\footnotesize,align=center] {C}
(b.west);
\draw[-latex] (a.south) -- node[below,font=\footnotesize] {D} (m2.west);
\draw[-latex] (m2.east) -- node[below,font=\footnotesize] {E} (b.south);
\end{tikzpicture}
\end{document}
