documentclass{article}

defmdiscretionary#1#2#3{%
{}$discretionary{hbox{$#1$}}{hbox{$#2$}}{hbox{$#3$}}${}}
begin{document}


aaadiscretionary{X}{Y}{Z}aaa

bigskip

parbox{1cm}{raggedright aaadiscretionary{X}{Y}{Z}aaa}

bigskip


$aaamdiscretionary{X}{Y}{Z}aaa$

bigskip

parbox{1cm}{raggedright $aaamdiscretionary{X}{Y}{Z}aaa$}



end{document}