I compiled the three suggested methods into one file to be able to compare them side by side. Note that setlength{leftmargin}{0pt} does not have any effect on the "enumerate" environment. So far, the best solution is the "list" environment using the option "leftmargin=1.4em". However, I do not like a constant number in my code for it makes the code fragile. Does anyone know how to compute this constant (1.4em) in terms of available LaTeX variables?

documentclass{article}
egin{document}

section*{Paragraph}
paragraph{1.} First
paragraph{2.} Second
paragraph{3.} Third

section*{list}


ewcounter{itemcounter}
egin{list}
{	extbf{arabic{itemcounter}.}}
{usecounter{itemcounter}leftmargin=1.4em}
item First
item Second
item Third
end{list}

section*{enumerate with leftmargin}
egin{enumerate}

enewcommand{labelenumi}{	extbf{	heenumi}.}
setlength{leftmargin}{0pt}
item First
item Second
item Third
end{enumerate}

end{document}