how to join lines in a diagram using tikz?

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shadows,positioning,shapes,arrows,decorations.markings,arrows.meta}

\tikzset{
      box/.style={draw,fill=white,rectangle,minimum size=2.5cm,align=center},
      diatostealth/.style={draw,{Diamond}-{Stealth}}
}

\begin{document}
\begin{tikzpicture}

\node[box](a){a}; 
\node[box, below left=1.5cm of a](b){b}; 
\node[box, below right=1.5cm of a](c){c}; 


% the arrows
\path[diatostealth] (a.south) -- (b.north){};
\path[diatostealth] (a.south) -- (c.north){};

\end{tikzpicture}
\end{document}

This is what I have currently.

This is what I am trying to achieve.