\definetabletemplate[booktabs][o0|l|l|ro0|]
\starttable[booktabs]
        \NC first column \NC second column \NC third column \NC \AR
\stoptable

Following example works:

\starttable[o0|l|l|ro0|] %same as above, but without the use of the defined template
        \NC first column \NC second column \NC third column \NC \AR
\stoptable

I know that this table-environment is deprecated. But it is best suited for my project. I love the use of the template with this environment.

Thanks for looking into it.

Christian