Enumerated list indentation

To get enumerated and itemized lists that have the same amounts of left-hand and right-hand indentation, I'd recommend using the enumitem package -- which provides many extensions to and improvements over LaTeX's list environments -- and its leftmargin and rightmargin options. The following MWE illustrates this.

documentclass{article}
usepackage[margin=3cm]{geometry} % per the example in your code
usepackage{lipsum} % for filler text
usepackage{enumitem}

begin{document}
lipsum[1] % filler text
begin{enumerate}[leftmargin=2cm,rightmargin=2cm]
item lipsum[2]
end{enumerate}
lipsum[3]
begin{itemize}[leftmargin=1cm,rightmargin=1cm]
item lipsum[4]
end{itemize}
end{document}

---

Added material after receiving a comment from the OP. I'm afraid I'm not quite sure about the meaning of the question

But how do I make the indentations of the item the same as the original text?

The new MWE below provides four separate option settings for shaping the itemized (or, equivalently, enumerated) list items. Hopefully, one of these settings meets your needs.

documentclass{article}
% some short filler text, ca. 3 lines long
newcommand{shortfiller}{Nam dui ligula, fringilla a, euismod sodales, sollicitudin vel, wisi. Morbi auctor lorem non justo. Nam lacus libero, pretium at, obortis vitae, ultricies et, tellus. Donec aliquet, tortor sed accumsan bibendum, erat ligula aliquet magna.}
usepackage{fancyvrb}
usepackage[margin=3cm]{geometry}
usepackage{enumitem}

begin{document} 
DefineShortVerb{|}
emph{``Normal'' text paragraph.} shortfiller
%% Option 1
begin{enumerate}[leftmargin=2cm, rightmargin=2cm]
item emph{Settings:} |[leftmargin=2cm, rightmargin=2cm]|. 
shortfiller
end{enumerate}

emph{``Normal'' text paragraph.} shortfiller
%% Option 2
begin{enumerate}[resume, wide=2cm, leftmargin=2cm, rightmargin=2cm]
item emph{Settings:} |[wide=2cm, leftmargin=2cm,  rightmargin=2cm]|.
shortfiller
end{enumerate}

emph{``Normal'' text paragraph.} shortfiller
%% Option 3
begin{enumerate}[resume, wide=parindent, leftmargin=parindent, rightmargin=parindent]
item emph{Settings:} |[wide=parindent, leftmargin=parindent, rightmargin=parindent]|.  
shortfiller
end{enumerate}

emph{``Normal'' text paragraph.} shortfiller
%% Option 4
begin{enumerate}[resume, wide=parindent]
item emph{Setting in this and the following item:} |[wide=parindent]|. 
shortfiller 
item shortfiller
end{enumerate}

emph{``Normal'' text paragraph.} shortfiller
end{document}

Suppose the final setting of the list items is what you're looking for. If you want this setting to apply uniformly to all enumerated and itemized lists, you should insert the command

setlist{wide=parindent}

in your document's preamble, immediately after the usepackage{enumitem} instruction.

Both this question as well as its (closed) duplicate How to move enumerate to the right suggest to use the enumitem environment.

However, it is known to conflict with beamer (in particular with the options [<+->], which you can pass on the list environments). Thus, in these cases, one needs a different solution:

I have solved this issue by simply putting the itemize into a minipage and moving the minipage to the right:

 begin{frame}{some title}
   
 hspace*{.1linewidth}%
 begin{minipage}[t]{.9linewidth}
   begin{itemize}[<+->]
     item first item
     item second item
   end{itemize}
 end{minipage}

 end{frame}

Please note that due to moving it to the right you might want to make the minipage smaller. Thus, when moving it 10% of the linewidth to the right (hspace*{.1linewidth}), make the minipage 90% of the linewidth (begin{minipage}[t]{.9linewidth}).

If you want "to see" how large your minipage is, surround it with fbox{}