documentclass{article}
usepackage{pgfplots}
usepackage{etoolbox}

begin{document}

    %Define macro
    defmyMacro{

        %Check for multipart variable definition
        ifdef{VariableOne}{
            ##1

            ##2

            ##3
        }{}

        #1
    }

    %Define multipart variable
    defVariableOne#1#2#3{
        {1}
        {2}
        {3}
    }
    myMacro{4}
end{document}

documentclass{article}
usepackage{pgfplots}
usepackage{etoolbox}


begin{document}
    defmyMacro#1{
        ifdef{VariableOne}{
            foreach x/y/z in VariableOne{
                x

                y

                z
            }
        }{}

        #1
    }

    defVariableOne{1/2/3}

    myMacro{4}
end{document}