Alignment in longtable

I have following:

\documentclass{scrbook}\usepackage{longtable,array,lipsum,tabularx,multirow}\newcolumntype{L}[1]{>{\raggedright\arraybackslash}p{#1}}\newcolumntype{R}[1]{>{\raggedleft\arraybackslash} p{#1}}\begin{document}\begin{longtable}[c]{b{20mm}|L{\dimexpr\textwidth-40mm\relax}@{}|R{20mm}}Column 1 & Column 2 & Column 3 \\ \hlinetop & \lipsum[1] & bottom\end{longtable}\end{document}

What I get is:

What I want is:

(contents of last column aligned at the bottom line)

Could someone help me to solve this?

Final solutionbased on this answer of @Zarko and on the comments of @DavidCarlisle

\documentclass{scrbook}\usepackage{lipsum,longtable, multirow, xltabular,letltxmacro}\RequirePackage[noprefix,refpage,intoc]{nomentbl}\newcolumntype{L}{>{\raggedright\arraybackslash}X}\LetLtxMacro\oldnomenclature\nomenclature\renewcommand{\nomenclature}[3][]{\oldnomenclature[#1]{#2}{#3}{}{}}\renewcommand{\pagedeclaration}[1]{\multirow[b]{\zzz}{*}{#1}}\makenomenclature\begin{document}\nomenclature{a}{\lipsum[1]}\nomenclature{b}{\lipsum[2]}\printnomenclature[3cm]\end{document}

used together with makeindex and the makeindex-Style-File nomentbl.ist which is

actual '@'quote '%'delim_0   ""delim_1   ""delim_2   ""item_0    ""delim_t   " \\\\\n"line_max  1000heading_prefix   "\\multicolumn{3}{@{}l}{\\nomgroup{"heading_suffix   "}} \\\\\n\\nopagebreak\\\\*[-3mm]\n\\nopagebreak{}"headings_flag       1group_skip        "\\\\*[-3mm]"preamble "\n\\begin{thenomenclature}\n%\\begin{xltabular}{\\linewidth}{@{}lL<{\\endgraf\\xdef\\zzz{\\the\\prevgraf}}@{}l@{}l@{}r@{}}\n%\\textbf{Symbol} & \\textbf{Description} & & & \\textbf{Page} \\\\*[-4mm]\n%\\hline \\\\*[-2mm] \n"%postamble "\n\\end{xltabular}\n\n\\end{thenomenclature}\n"keyword "\\nomenclatureentry"\endinput

results in