Triple lines in TikZ

Question

I need to be able to draw tripled paths in TikZ. I guess this could be done by some clever style which draws a big doubled path to get the outer curves correct, then uses some clever post-action hackery to draw a second non-doubled path to produce the central curve. I need to be able to set the thickness and the spacings of the three lines independently, just like one can with doubled lines.

Luckily, I don't need arrowheads on the line!

Anyone up to the challenge? :)

Answer 1

So basically a pre action on the path. I have added a style which is shown how to be called, notice that it will always draw.

\documentclass{article}
\usepackage{tikz}
\begin{document}
\tikzset{
    triple/.style args={[#1] in [#2] in [#3]}{
        draw,#1,preaction={draw,#2,preaction={draw,#3}}
    }
}        
\begin{tikzpicture}
  \draw[triple={[line width=1mm,red] in
      [line width=2mm,blue] in
      [line width=4mm,black]}] (0,0) to[out=10,in=100] (0,1);
\end{tikzpicture}
\end{document}

Answer 2

\documentclass[pstricks,border=12pt]{standalone}
\usepackage{pstricks}


\begin{document}


\begin{pspicture}(5,5)
    \rput(2.5,4){Step 1}
    \psline[linewidth=5pt](5,5)
\end{pspicture}


\begin{pspicture}(5,5)
    \rput(2.5,4){Step 2}
    \psline[border=1pt,bordercolor=yellow,linewidth=1pt](5,5)
\end{pspicture}


\begin{pspicture}(5,5)
    \rput(2.5,4){Step 3}
    \psline[linewidth=5pt](5,5)
    \psline[border=1pt,bordercolor=yellow,linewidth=1pt](5,5)
\end{pspicture}

\end{document}