Change indentation for the first item only

Question

For my own use (consisting in putting a box before the first item !), I want to change the indentation of the first item, something like this file:

I obtained this result, by the ugly method in the .tex file what I add here, just to watch up the result I want to obtain:

\documentclass[12pt]{article}
\newcounter{ItemsCounter}
\begin{document}
\pagestyle{empty}
\noindent
\textbf{Hello world}
\begin{list}
{\bfseries{}\arabic{ItemsCounter}.~}
{
\usecounter{ItemsCounter}
\addtolength\leftmargin{14mm}
}
\item A\\ multilined\\ text

\end{list}

\begin{list}
{\bfseries{}\arabic{ItemsCounter}.~}
{
\usecounter{ItemsCounter}
\setcounter{ItemsCounter}{1}}
\item Item two
\item Item three

\end{list}
\end{document}

I want to change only the first item by something like the command \addtolength\leftmargin{14mm}, but it's accepted only in global option and not locally for one item. I used this acrobaty, using two successive list environments, and iterating the items, to obtain the suitable result, but I obviously want to use a more elegant method, for enumerate and itemize environments. How can I obtain the compilation here, so change the indentation only for the first item, with a useful method ? Thank you.

Answer 1

Perhaps something like this?

\documentclass{article}
\usepackage{enumitem}
\begin{document}
\begin{enumerate}[label=\bfseries{\arabic{enumi}}.,leftmargin=1em]
\item[]\stepcounter{enumi}
\begin{enumerate}[label=\bfseries{\arabic{enumi}}.,leftmargin=3em]
\item A\\ multilined\\ text
\end{enumerate}
\item Item two
\item Item three
\end{enumerate}
\end{document}

You can adjust the identitation by playing with the leftmargin parameters.