Use Beamer to put 3 slides per page with notes beside (like powerpoint)

I am not sure, if this is what you are looking for, but you can tell pgfpages how many frames you'd like to have on one page.

[In case this is not what you are asking, can you please include a screenshot or similar in your question of what the output should look like?]

\documentclass[11pt]{beamer}

\usetheme{Warsaw}

\author{Christopher Donham}
\date{Fall 2017}
\institute{Community College of Baltimore County}

\setbeameroption{show notes}
\usepackage{pgfpages}
\pgfpagesdeclarelayout{6 on 1}
{
    \edef\pgfpageoptionheight{\the\paperwidth} % landscaped by default
    \edef\pgfpageoptionwidth{\the\paperheight}
    \def\pgfpageoptionborder{0pt}
}
{
    \pgfpagesphysicalpageoptions
    {%
        logical pages=6,%
        physical height=\pgfpageoptionheight,%
        physical width=\pgfpageoptionwidth%
    }
    \pgfpageslogicalpageoptions{1}
    {%
        border shrink=\pgfpageoptionborder,%
        resized width=.5\pgfphysicalwidth,%
        resized height=.25\pgfphysicalheight,%
        center=\pgfpoint{.29\pgfphysicalwidth}{.8333\pgfphysicalheight}%
    }%
    \pgfpageslogicalpageoptions{2}
    {%
        border shrink=\pgfpageoptionborder,%
        resized width=.5\pgfphysicalwidth,%
        resized height=.25\pgfphysicalheight,%
        center=\pgfpoint{.75\pgfphysicalwidth}{.8333\pgfphysicalheight}%
    }%
    \pgfpageslogicalpageoptions{3}
    {%
        border shrink=\pgfpageoptionborder,%
        resized width=.5\pgfphysicalwidth,%
        resized height=.25\pgfphysicalheight,%
        center=\pgfpoint{.29\pgfphysicalwidth}{.5\pgfphysicalheight}%
    }%
    \pgfpageslogicalpageoptions{4}
    {%
        border shrink=\pgfpageoptionborder,%
        resized width=.5\pgfphysicalwidth,%
        resized height=.25\pgfphysicalheight,%
        center=\pgfpoint{.75\pgfphysicalwidth}{.5\pgfphysicalheight}%
    }%
    \pgfpageslogicalpageoptions{5}
    {%
        border shrink=\pgfpageoptionborder,%
        resized width=.5\pgfphysicalwidth,%
        resized height=.25\pgfphysicalheight,%
        center=\pgfpoint{.29\pgfphysicalwidth}{.1667\pgfphysicalheight}%
    }%
    \pgfpageslogicalpageoptions{6}
    {%
        border shrink=\pgfpageoptionborder,%
        resized width=.5\pgfphysicalwidth,%
        resized height=.25\pgfphysicalheight,%
        center=\pgfpoint{.75\pgfphysicalwidth}{.1667\pgfphysicalheight}%
    }%   
}
\pgfpagesuselayout{6 on 1}[a4paper,border shrink=7mm]
\begin{document}

\title[Math 153 - Day 6]{Normal Distributions}

\begin{frame}
\note{page 1}
\titlepage
\end{frame}

\begin{frame}
\note{page 2}
\titlepage
\end{frame}

\begin{frame}
\note{page 3}
\titlepage
\end{frame}

\end{document}

EDIT 2019:

With pgf version 3.1 or newer (available from https://sourceforge.net/p/pgf/git/ci/master/tree/), the above code can be simplified to

\documentclass[11pt]{beamer}

\usetheme{Warsaw}

\author{Christopher Donham}
\date{Fall 2017}
\institute{Community College of Baltimore County}

\setbeameroption{show notes}
\usepackage{pgfpages}

\pgfpagesuselayout{6 on 1}[a4paper,border shrink=7mm]
\begin{document}

\title[Math 153 - Day 6]{Normal Distributions}

\begin{frame}
\note{page 1}
\titlepage
\end{frame}

\begin{frame}
\note{page 2}
\titlepage
\end{frame}

\begin{frame}
\note{page 3}
\titlepage
\end{frame}

\end{document}

I don't know how to extract notes from the original document, but if you accept to work with two files: presentation and notes, the one with notes could be based in sidebyside tcolorboxes where one side is a slide from a previously compiled beamer presentation while you can write notes on the other side.

Following code defines \mypage environment with one default and one mandatory parameters. The default is the name of your presentation file (change beamer with any other name). The mandatory parameter is the number of the slide that you want to comment. The environment's body will the notes corresponding to the slide. It doesn't matter if notes are long or short, they will fit inside a box and every page will contain as many boxes as possible.

\documentclass{article}
\usepackage[most]{tcolorbox}
\usepackage{lipsum}

\newenvironment{mypage}[2][beamer]{%
\begin{tcolorbox}[%
    fonttitle=\bfseries, 
    title={Notes to slide #2}, 
    sidebyside,
    sidebyside align=top seam,
    bicolor, 
    ]
\includegraphics[page=#2, width=\linewidth]{#1}
\tcblower}{\end{tcolorbox}}

\begin{document}

\begin{mypage}{1} 
This is a short comment
\end{mypage}

\begin{mypage}{2}
\end{mypage}

\begin{mypage}{3}
\lipsum[2]
\end{mypage}
\end{document}

If you don't want to "waste" paper and ink, use a blankest tcolorbox:

\newenvironment{mypage}[2][beamer]{%
\begin{tcolorbox}[%
    blankest, 
    sidebyside,
    sidebyside align=top seam,
    ]
\includegraphics[page=#2, width=\linewidth]{#1}
\tcblower}{\end{tcolorbox}}