\documentclass{article}
\usepackage{xparse}
\usepackage{expl3}
\def\SpecialDirName{special}%
\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\GetFileName}{%
O{default}%
m%
m%
m%
}{%
\str_if_eq:xxTF{#1}{\SpecialDirName}{#4}{#2-#3-#4}%
}%
\ExplSyntaxOff
\begin{document}
\edef\FileName{\GetFileName{a}{b}{c}}
FileName = "\FileName".
\edef\FileName{\GetFileName[special]{a}{b}{c}}
FileName = "\FileName".
\end{document}