Command to make comma separated commands

This doesn't check for list length, but that can be done. Likewise, the arguments can be added to a \foreachitem loop.

\documentclass{article}
\usepackage{listofitems}
\newcommand\test[1]{%
  \readlist*\mylist{#1}%
  \mylist[1] is \mylist[2] a \mylist[3].
}
\begin{document}
\test{This, just, test}
\end{document}

Here's an example where number of arguments may vary, yet be accounted for:

\documentclass{article}
\usepackage{listofitems}
\newcommand\test[1]{%
  \readlist*\mylist{#1}%
  There are \mylistlen{} arguments:
  \foreachitem \z \in \mylist[]{\ifnum\zcnt=1\relax\else, \fi(\zcnt) \z}.
}
\begin{document}
\test{This, just, test}

\test{This, just, test, today, Hallelujah}
\end{document}

I figured it out eventually:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\newcommand{\newcscommand}[3]{
  \NewDocumentCommand{ #1 }{ >{\SplitArgument{#2 - 1}{,}}m }{\csname aux\cs_to_str:N #1\endcsname##1}
  \expandafter\newcommand\csname aux\cs_to_str:N #1\endcsname[#2]{#3}
}

\ExplSyntaxOff

\newcscommand{\test}{3}{#1 is #2 a #3.}
\begin{document}
\test{This, just, test}
\end{document}

Your idea is good, but proper expl3 programming is better:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand{\newcscommand}{mO{0}m}
 {
  \NewDocumentCommand{#1}{ >{\SplitArgument{#2-1}{,}}m }
    {
     \use:c { __noibe_\cs_to_str:N #1:\prg_replicate:nn{#2}{n} } ##1
    }
  \cs_new:cn { __noibe_\cs_to_str:N #1:\prg_replicate:nn{#2}{n} } { #3 }
 }

\ExplSyntaxOff

\newcscommand{\test}[3]{#1 is #2 a #3.}

\begin{document}

\test{This, just, test}

\end{document}

Note that the syntax of \newcscommand is similar to \newcommand.

The trick is indeed to create a macro with the correct number of arguments in its signature.

A variant that avoids calling \use:c and \prg_replicate:nn each time \test is executed, because the control sequence name is built at definition time thanks to \exp_not:c.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand{\newcscommand}{mO{0}m}
 {
  \exp_args:Nnne \NewDocumentCommand{#1}{ >{\SplitArgument{#2-1}{,}}m }
   {
    \exp_not:c { __noibe_\cs_to_str:N #1:\prg_replicate:nn{#2}{n} } ##1
   }
  \cs_new:cn { __noibe_\cs_to_str:N #1:\prg_replicate:nn{#2}{n} } { #3 }
}
\exp_args_generate:n { nne }

\ExplSyntaxOff

\newcscommand{\test}[3]{#1 is #2 a #3.}

\begin{document}

\test{This, just, test}

\end{document}