# Problems at Plotting the function in 3d System

I want to plot the function x=-3 in a 3D System. I've been searching in the internet for hours and found this example :

\begin{tikzpicture}[
declare function = {
X1(\y,\z) = -3;
}]
\begin{axis}[grid,
x={(-0.7071cm,-0.7071cm)},
y={(1cm,0.0cm)},
z={(0cm,1cm)},
axis lines=center,
font=\footnotesize,
xmax=5.4,ymax=5.4,zmax=5.4,
xmin=-5.4,ymin=-5.4,zmin=-5.4,
xlabel=\normalsize$x$,ylabel=\normalsize$y$,zlabel=\normalsize$z$,
major tick style = {black},
minor tick num=1, minor tick style = {very thin},
axis line style = {-latex}, %Pfeilspitzen
%enlargelimits=0.1

% Ebene x=-3
\end{axis}
\end{tikzpicture}


The example I got this example was z=5, now I wanted to plot this new Code, but when it is not working. when I put z anywhere there's the following Package PGF Math Error could not parse '' as a floating point number. Is it possible, that the System doesn't see z as a variable ? I don't have any other ideas ... Thanks for help

For 3d-plots you need points with three coordinates; {X1(y,z)} is just one component. Moreover, the varying parameters are called x and y. \documentclass[border=2mm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.14}
\begin{document}
\begin{tikzpicture}%
[declare function = { X1(\y,\z) = -3;} ]
\begin{axis}%
[grid,
% x={(-0.7071cm,-0.7071cm)},
% y={(1cm,0.0cm)},
% z={(0cm,1cm)},
axis lines=center,
font=\footnotesize,
xmax=5.4,ymax=5.4,zmax=5.4,
xmin=-5.4,ymin=-5.4,zmin=-5.4,
xlabel={\normalsize$x$},
ylabel={\normalsize$y$},
zlabel={\normalsize$z$},
major tick style = {black},
minor tick num=1, minor tick style = {very thin},
axis line style = {-latex},
]