documentclass{article}
usepackage{titlesec}

defbl#1 #2 {#1textsuperscript{#2},}
titleformat{section}[runin]{}{}{0pt}{}[. ]
%titleformat{section}[runin]{}{}{0pt}{bl}[. ]

begin{document}
section{3 a 21}
main text.
end{document}