Question about "tikzpicture"

Question

I have a question regarding using "tikzpicture". I intend to draw a figure, which includes a top rectangular, middle circle and bottom rec. I tried and drew what I wanted with considering coordination for each shape. But I would like to consider the position of each shape depending on the position of the upper shape. I coded what I want but there is an intersection between my shaps.

Here is my code:

\documentclass{article}
 \usepackage{tikz}
 \usepackage{pgfplots}
\begin{document}
\tikzstyle{cir}=[circle,draw=blue!50,fill=blue!20,thick,
inner sep=0pt,minimum size=3cm]
\tikzstyle{rec}=[rectangle,draw=black!50,fill=black!20,thick,
    inner sep=0pt,minimum size=3cm]
\begin{figure}
   \centering
   \begin{tikzpicture}
     \node[rec] (top_rec)                            {};
     \node[cir] (middle_cir) [below of=top_rec]      {};
     \node[rec] (down_rec)   [below of=middle_cir]   {};
     \end{tikzpicture}   
  \caption{Caption}
  \label{fig:my_label}
\end{figure}
\end{document}

Here also is what I got

Answer 1

Is this what you want?

Observe that I changed the anchor in the node style and changed their positions making use of at and passing the anchor explicitly.

\documentclass{article}
 \usepackage{tikz}
 \usepackage{pgfplots}
\begin{document}
\tikzstyle{cir}=[circle,draw=blue!50,fill=blue!20,thick,
inner sep=0pt,minimum size=3cm,anchor=north] % added anchor
\tikzstyle{rec}=[rectangle,draw=black!50,fill=black!20,thick,
    inner sep=0pt,minimum size=3cm,anchor=north] % added anchor
\begin{figure}
   \centering
   \begin{tikzpicture}[]
     \node[rec] (top_rec)                            {A};
     \node[cir] (middle_cir) at (top_rec.south)      {B}; % changed
     \node[rec] (down_rec)   at (middle_cir.south)   {C}; % changed
   \end{tikzpicture}
   \caption{Caption}
   \label{fig:my_label}
\end{figure}
\end{document}

Answer 2

With use of the chains and positioning library, and defining nodes style as options of `tikzpicture:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{chains,
                positioning}
\begin{document}
    \begin{figure}
   \centering
   \begin{tikzpicture}[
node distance = 0pt,
  start chain = going below,
     C/.style = {circle, draw=blue!50,thick, fill=blue!20,
                 inner sep=0pt, minimum size=3cm,
                 on chain},
     R/.style = {draw=black!50, thick, fill=black!20,
                 inner sep=0pt, minimum size=3cm,
                 on chain}
                        ]
\node[R] (n1)   {};
\node[C] (n2)   {};
\node[R] (n3)   {};
     \end{tikzpicture}
\caption{Caption}
\label{fig:my_label}
    \end{figure}
\end{document}

Result of compilation is:

Addendum: Regarding your comment with your new request:

• Please don't add some code to my answer which change it. If you have new problem, please ask new question, if you like to clarify your problem, edit your question and describe it there, that it can be seen to other members of site.
• Arrows with labels between rectangles in your images you can draw as edges with quotes. See MWE below.
• In MWE below is changed nodes style declaration too. I do this to show one more possibilities how to define it and that they support your new request in comment.
• Style and text of quotes you can change according to your needs.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows.meta,
                chains,
                positioning,
                quotes}
\begin{document}
    \begin{figure}
   \centering
   \begin{tikzpicture}[
node distance = 0pt,
  start chain = going below,
  base/.style = {draw=#1!50, thick, fill=#1!20,
                 inner sep=0pt, outer sep=0pt, minimum size=3cm,
                 on chain}, % <--- new 
     C/.style = {circle, base=blue},    % <--- changed
     R/.style = {base=black},           % <--- changed
       every edge/.style = {draw, -Straight Barb, semithick, bend angle=45},    % <---  for edges
every edge quotes/.style = {auto=right, font=\small\bfseries}   % <--- for quotes on edges
                        ]
\node[R] (n1)   {};
\node[C] (n2)   {};
\node[R] (n3)   {};
% edges
\draw   (n1.west)  edge [bend right, "A"] (n3.west)
        (n3.east)  edge [bend right, "B"] (n1.east);
     \end{tikzpicture}
\caption{Caption}
\label{fig:my_label}
    \end{figure}
\end{document}