[Tex/LaTex] A leading zero in the enumerate environment

Question

I need to make my Enumerate look like this:

REQ/Text/001
REQ/Text/002
…

What i can do is this:

REQ/Text/1
…

But how can i add leading Zeros? I tried the answer in:
Enumerate list – numbers with prefix
but it doesnt work. It brings me something like:

REQ/Text/1!100 01!10 001

but the exclamation mark is turned around.
Sorry for my bad English, im from Germany.

Thanks.

Edit:
A minimal Code example is:

    \documentclass[12pt, a4paper]{article}

    \usepackage{german,epsfig,longtable,graphicx,fullpage,float}%

    \usepackage[ansinew]{inputenc} % Umlaute (Windows)

    \usepackage{enumitem}

    \def\threedigits#1{% 
    \ifnum#1<100 0\fi 
    \ifnum#1<10 0\fi 
    \number#1}

    \begin{document}

    \begin{enumerate}[label=\textbf{RQ/Gru/\threedigits{\theenumi}}, leftmargin = *]
        \item \label{anfo:norm:S1}  
            \item \label{anfo:norm:S2}  
            \item \label{anfo:norm:S3}  
            \item \label{anfo:norm:S4}
    \end{enumerate}

    \end{document}

This minimal example produces the Error.

Error Messages are:

Missing number, threated as zero

Missing = inserted for \ifnum

Answer 1

You miss the \protect command before using the \threedigits one.

\documentclass[12pt, a4paper]{article}

\usepackage{german,epsfig,longtable,graphicx,fullpage,float}%

\usepackage[ansinew]{inputenc} % Umlaute (Windows)

\usepackage{enumitem}

\def\threedigits#1{% 
\ifnum#1<100 0\fi 
\ifnum#1<10 0\fi 
\number#1}

\begin{document}

\begin{enumerate}[label={\textbf{RQ/Gru/\protect\threedigits{\theenumi}}}, leftmargin = *]
    \item \label{anfo:norm:S1}  
        \item \label{anfo:norm:S2}  
        \item \label{anfo:norm:S3}  
        \item \label{anfo:norm:S4}
\end{enumerate}

\end{document}