documentclass[12pt]{report}
usepackage[utf8]{inputenc}
usepackage{amssymb}
usepackage{nomencl,etoolbox,ragged2e,siunitx}

newcommand{insertnomheaders}{item[bfseries Symbol]%
textbf{Description}hfillmakebox[6em]{hfill}{textbf{Dimension}}}

renewcommandnomgroup[1]{%
item[bfseries
ifstrequal{#1}{A}{Acronyms}{%
ifstrequal{#1}{R}{Roman Symbols}{%
ifstrequal{#1}{G}{Greek Symbols}{%
ifstrequal{#1}{O}{Other Symbols}{}}}}]
insertnomheaders
}

newcommand{nomunit}[1]{%
renewcommand{nomentryend}{hspace*{fill}#1}}

makenomenclature

begin{document}

nomenclature[A]{DE}{Differential Equation}
nomenclature[R]{$r$}{Rain Intensity plus a very very very very very very very very very very very long description nomunit{$frac{L}{T}$}}

printnomenclature[6em]

end{document}