Misplace omit in multicolumn alias definition

I’m attempting to make my syntax cleaner and more readable (to me) by making aliases for some commands. One is the mC command, which produces a multicolumn with removed left and right padding. When invoking the command, I get a misplaced omit error. Any ideas?

PS: is there a non-risky way to redefine tabular to default to @{} at the ends of the tabular column specification (but not in between…)?

MWE:

documentclass{article}

usepackage{xparse}
usepackage{multirow}
usepackage{makecell}
usepackage{booktabs}

NewDocumentCommand{mC}{m O{c} m}{% multicolumn cell
    multicolumn{#1}{@{}#2@{}}{#3}%
}%

NewDocumentCommand{lbC}{O{t} O{l} m}{% line-break cell
    makecell[{#1}{#2}]{#3}%
}%


begin{document}
    begin{tabular}{@{}lll@{}}
        lbC[b][l]{L1L2} & a & b
        mC{2}[l]{Multi col}  & 3  % causes misplaces omit
        multicolumn{2}{@{}l@{}}{Multi col}  & 3   % works fine
    end{tabular}

end{document}

documentclass{article}
newcommandmC[3]{multicolumn{#1}{@{}#2@{}}{#3}}%
newenvironment{mytabular}[1]{%
  begin{tabular}{@{}#1@{}}%
}{%
  end{tabular}%
}
begin{document}
    begin{mytabular}{lll}
      mC{2}{l}{Multirow}  & 3 
      multicolumn{2}{@{}l@{}}{Multirow}  & 3 
    end{mytabular}
end{document}