\documentclass{article}
\begin{document}
\ExplSyntaxOn
\keys_define:nn {path}{A .code:n = A#1,A .groups:n = GA}
\keys_define:nn {path}{B .code:n = B#1,B .groups:n = GB}
\keys_define:nn {path}{C .meta:n = {A=#1,B=#1}, C .groups:n={GA}}
\keys_set_groups:nnn {path}{GA}{A=a,B=b,C=c}
\ExplSyntaxOff
\end{document}