How to label subfigures in Tikz package?

Making labeled subfigures is a bit tricky using the tikz package. Could someone please show me how to provide each subfigure its own letter (A for the left subfigure, B for the middle figure, and C for the right subfigure)? The code is below:

\documentclass[aps,amsmath,amssymb,letter,scriptaddress,twocolumn, prl,showkeys]{revtex4}

\usepackage{amsmath}
\usepackage{amsthm}
\newtheorem{thm}{Theorem}
\usepackage{amssymb}
\usepackage{pgfplots}
\usepgfplotslibrary{groupplots}
\usepackage{mathtools}
\usepackage{makeidx}
\usepackage{amsfonts}
\usepackage[ansinew]{inputenc}
\usepackage[usenames,dvipsnames]{pstricks}
\usepackage{subfigure}
\usepackage{epsfig}
\usepackage{pst-grad} % For gradients
\usepackage{pst-plot} % For axes
\usepackage[colorlinks,hyperindex]{hyperref}
\makeatletter
\newcommand{\rmnum}[1]{\romannumeral #1}
\newcommand{\Rmnum}[1]{\expandafter\@slowromancap\romannumeral #1@}
\makeatother

\begin{document}

\begin{figure*}
    \begin{tikzpicture}[font=\footnotesize\sffamily]
      \begin{groupplot}[
         group style={group size=3 by 1,
    ylabels at=edge left
},
      view={0}{90},
      width=5cm,
      height=5cm,
      scale only axis,
      xmin=0, xmax=3,
      ymin=-8, ymax=0,
xlabel={x},
    ylabel={$y$},
      name=plot2,
      unbounded coords=jump]
    ]
    \nextgroupplot         
\addplot [only marks, mark=o, color=red] table{
1       -2.5
2       -3.3
3       -3.4
};
\addplot [domain=1:3, color=black] {-0.0538*x-2.8234};
        \nextgroupplot         
\addplot [only marks, mark=o, color=red] table{
1       -2.900422094
2       -3.244193633
3       -3.283414346
};
\addplot [domain=4:6, color=black] {-0.0333*x-3.1637};
        \nextgroupplot         
\addplot [only marks, mark=o, color=red] table{
4       -4.199705078
5       -4.674163057
6       -4.674163057
};
\addplot [domain=4:6, color=black] {-0.0749*x-1.7959};
 \end{groupplot}
    \end{tikzpicture}
\caption{My caption} 
 \label{myfig}
  \end{figure*}

\end{document}