Explain how you would name a sorting rule for 1 square , 1 rectangle and 1 triangle

A common characteristic for the shapes given is that they all consist of sides. Therefore, the most applicable sorting rule is the number of sides. In this regard, there will be two categories of the shapes given.

That is, number of sorting rule in which:
Category 1: 1 square and 1 rectangle all with four sides.
Category 2: 1 triangle with three sides.