Action based upon optional argument in \NewDocumentEnvironment

Question

With the following environment, I would like to make #3 an optional argument. In a way that when #3 is present, the text starts on the next line. And when #3 is missing, the text is written in the adjacent area as done by the theorem environment.

\NewDocumentEnvironment{Pz}{mmm}
  %% General Proposition
  %% \Pz {SPECIFIER} {LOCATOR} {TITLE}
  { \noindent #1 \ #2 \ (#3) }
  {}

\begin{document}

\maketitle
\tableofcontents

\chapter {This is an introduction
  to some topic}

\section{Some Section}

This section is about this and that.

\begin{Pz}{Theorem}{GCHQ}{Pythagoras}
  Some Text
\end{Pz}

\begin{Pz}{Theorem}{GCHQ}
  Some Text on new line
\end{Pz}

\end{document}

The following is not doing the trick

\NewDocumentEnvironment{Pz}{mmO{}}
  %% General Proposition
  %% \wvPz {SPECIFIER} {LOCATOR} {TITLE}
  %% TITLE Optional, Default Empty
  { \noindent #1 \ #2 \ (#3)
    \IfValueT {#3} { \newline }
  }
  {}

Answer 1

Something like this. Optional arguments are represented by the o specifier, (or O{default} if you want an optional argument with a default value. You can then test whether the argument was provided by using the \IfNoValueTF{no arg}{arg} test, or one of its alternatives.

\documentclass{article}

\NewDocumentEnvironment{Pz}{mmo}
{\IfNoValueTF{#3}
    {\noindent #1 \ #2  }%
    {\noindent  #1 \ #2 \ (#3)\par}%
    }
    {}
\begin{document}

\begin{Pz}{Theorem}{GCHQ}[Pythagoras]
  Some Text
\end{Pz} 

\begin{Pz}{Theorem}{GCHQ}  Some Text
\end{Pz} 

\end{document}