To find out how much time it would take for each group to tell their stories, we simply need to multiply the amount of people in each group by the amount of time it takes one member to tell a story.
We know that there are either 3 or 4 people in each group, and that it takes 3 minutes for each person to tell their stories, so we simply need to multiply
3 * 3 = 9
4 * 3 = 12
It will take either 9 or 12 minutes for each group to tell their scary stories.
Hope that helped! =)