Q:

\item that starts with a number doesn't work with style=nextline

I've got a list of items and use style=nextline.  Problem is, when a list-item starts with a number, the nextline doesn't work:
\begin{example}[style=nextline]
\item[3D]
    Doesn't line-break before this text. 
\item[Three D]
    Line-breaks before this text. 
\end{example}

Any ideas? 

A:

I assume your example environment is defined via enumitem, but I can't guess how.  The manual is clear about style=nextline: it won't issue a line break if the label fits in the left margin.  So you need to make the label grow wider. 
I used geometry only to show the margins. 
\documentclass{article}
\usepackage[showframe,pass]{geometry}
\usepackage{enumitem}
\newenvironment{example}
  {\renewcommand{\descriptionlabel}[1]{%
     \hspace\labelsep\makebox[\columnwidth][l]{\normalfont\bfseries ##1}%
   }\description[style=nextline]}
  {\enddescription}

\begin{document}
\begin{example}
\item[3D]
    Doesn't line-break before this text. 
\item[Three D]
    Line-breaks before this text. 
\end{example}
\end{document}

