Which of the following statements regarding sulphur trioxide is not true ?

Question

$\begin{array}{1 1}\text{Sulphur trioxide exists in three polymorphic forms}\\\text{Sulphur trioxide is an anhydride of sulphuric acid}\\\text{Vanadium pentoxide is used as a catalyst in converting sulphur dioxide into sulphur trioxide}\\\text{Sulphur trioxide has tetrahedral geometry}\end{array}$