The structure of $3$-pyramidal groups

A combinatorial block design $D$ is called $3$-pyramidal if there exists a subgroup $G$ of $Aut(D)$ fixing $3$ points and acting regularly on the other points. If this happens, we say that the design is $3$-pyramidal under $G$. In case $D$ is a Kirkman triple system, it is known that such a group $G$ has precisely $3$ involutions, all conjugate to each other. In this paper, we obtain a classification of the groups with this property.