How to draw this diagram with three boxes in TikZ?

Question

My goal is to make a Tikz picture that looks like the example with the white boxes and letters inside.

This is what I did so far (my example has gray boxes). I am not sure how to stack it evenly as in the picture with the white boxes. My LaTeX code is at the bottom... I would then like to output the figure as an image file like a jpeg or tif or png.

 \begin{tikzpicture}[>=latex]
 \tikzstyle{rect}=[draw=black, 
                   rectangle, 
                   fill=gray,
                   fill opacity = 0.2,
                   text opacity=1,
                   minimum width=100pt, 
                   minimum height = 50pt, 
                   align=center]
  \node[rect] (a1) {A};
  \node[rect,below right=of a1] (a3) {C};
  \node[rect,above right=of a1] (a2) {B};
\draw[->] (a1.north)--(a2.west)node[midway,sloped, above,xshift=-2mm]{1};
\draw[->] (a1.south)--(a3.west)node[midway,sloped, below,xshift=-2mm]{2};
\draw[->] (a2.south)--(a3.north)node[midway, xshift=6mm]{3};
    \end{tikzpicture}
    \end{figure}

Answer 1

Here is one out of many ways to arrange the nodes like this. Note also that \tikzstyle is deprecated.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{figure}
\begin{tikzpicture}[>=latex,rect/.style={draw=black, 
                   rectangle, 
                   fill=gray,
                   fill opacity = 0.2,
                   text opacity=1,
                   minimum width=100pt, 
                   minimum height = 50pt, 
                   align=center}]
  \node[rect] (a1) {A};
  \node[rect,right=100pt of a1] (a2) {B};
  \path (a1) -- (a2) node[midway,below=60pt,rect] (a3) {C};
  \draw[->] (a1)--(a2)node[midway,sloped, above]{1};
  \draw[->] (a1.south)--(a3.west)node[midway,sloped,below]{2};
  \draw[->] (a2.south)--(a3.east)node[midway,sloped,below]{3};
\end{tikzpicture}
\end{figure}
\end{document}

Answer 2

Another way, with added arrows.meta and quotes TikZ libraries:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows.meta,
                positioning,
                quotes}

\begin{document}
    \begin{figure}
\begin{tikzpicture}[
node distance= 9mm and 0mm,
rect/.style = {draw=black, fill=gray!20, %font=\LARGE,
               minimum width=100pt, minimum height = 50pt},
every edge/.style = {draw, -Latex},
every edge quotes/.style = {sloped, auto=right}
                ]
\node[rect] (a) {A};
\node[rect,below right=of a] (c) {C};
\node[rect,above right=of c] (b) {B};
%
\path   (a) edge["1"  swap] (b)
        (a.south) edge["2"] (c.west)
        (b.south) edge["3"] (c.east);
\end{tikzpicture}
    \end{figure}
\end{document}