Two Girls, One Mother

Riddle: Two girls were born to the same mother, on the same day, at the same time, in the same month and year and yet they're not twins. How can this be?

Answer: They are a set of triplets. Source: https://riddles.com/4176

Two girls were born to the same mother, on the same day, at the same time, in the same month and year and yet they're not twins. How can this be? — Two Girls, One Mother by Dilan jose Riddle