\usepackage{enumitem}

\makeatletter
\newcommand*{\triviaenum}[1]{%
  \expandafter\@triviaenum\csname c@#1\endcsname%
}

\newcommand*{\@triviaenum}[1]{%
  $\ifcase #1 \or 1. \or \textrm{X}. \or 2. \or \textrm{\O}. %
    \else\@ctrerr\fi$%
}
\AddEnumerateCounter{\triviaenum}{\@triviaenum}{3}
\makeatother

% Ø.%