RequirePackage{filecontents}
begin{filecontents}{test.bib}
@article{author1,
journal = {journal},
year = {1999},
author = {author1},
}

@article{author2,
  journal = {journal},
year = {1999},
author = {author2},
}

@article{author3,
journal = {journal},
year = {1999},
author = {author3},
}

end{filecontents}
documentclass[12pt,a4paper, headinclude=true, titlepage=true,DIV=12,oneside,BCOR=1cm, numbers=noendperiod, toc=bib, chapterprefix=true]{scrreprt}

usepackage[babel,german=quotes]{csquotes}
usepackage[backend=biber,citestyle=numeric-comp,bibstyle=chem-angew,mcite=true,subentry,pageranges=true,sorting=none,citetracker]{biblatex}
addbibresource{my_refs.bib}
usepackage[ngerman,KeepShorthandsActive]{babel}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}

addbibresource{test.bib}

begin{document}

Here I cite a reference with several    subentries.defbibentryset{set1}{author1,author2,author3}supercite{set1} Somewhere later, I want to refer to some of the subentries.supercite{author1,author2}

printbibliography

end{document}

RequirePackage{filecontents}
begin{filecontents}{jobname.bib}
@article{author1,
  journal = {journal},
  year = {1999},
  author = {author1},
}

@article{author2,
  journal = {journal},
  year = {1999},
  author = {author2},
}

@article{author3,
  journal = {journal},
  year = {1999},
  author = {author3},
}
end{filecontents}
documentclass[12pt,a4paper, headinclude=true, titlepage=true,DIV=12,oneside,BCOR=1cm, numbers=noendperiod, toc=bib, chapterprefix=true]{scrreprt}
usepackage[ngerman,KeepShorthandsActive]{babel}
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage[babel,german=quotes]{csquotes}
usepackage[backend=biber, citestyle=numeric-comp, bibstyle=chem-angew, mcite=true, subentry, pageranges=true, sorting=none, citetracker]{biblatex}

addbibresource{jobname.bib}

makeatletter
renewbibmacro*{cite:comp:inset}{%
  usebibmacro{cite:dump}%
  ifnumgreater{value{cbx@tempcntb}}{-1}
    {multicitedelim}
    {}%
  printtext[bibhyperref]{%
    iffieldequals{labelnumber}{cbx@lastnumber}
      {}
      {printfield{labelprefix}%
       printfield{labelnumber}}%
    printfield{entrysetcount}}%
  savefield{labelnumber}{cbx@lastnumber}%
  setcounter{cbx@tempcntb}{-1}}
makeatother


begin{document}
Here I cite a reference with several subentries.defbibentryset{set1}{author1,author2,author3}supercite{set1} Somewhere later, I want to refer to some of the subentries.supercite{author1,author2,author3}

printbibliography
end{document}