#1: If all four sides are equal, it still may not be a square. For example, if all angles are not 90* then it is a Rhombus. Not sufficient.
#2: Three angles 90* means the fourth angle has to be 90* too. However, if a quadrilateral has all four angles equal to 90*, does it mean it is a square? No.
It could be a rectangle. Not sufficient.
Combining #1 and #2: The quadrilateral must have all four angles 90*. Since 3 sides are equal, the fourth one must be equal to these three since the pair of opposite sides must be equal if all four angles are 90*. Hence, the quadrilateral must be a square.
Hope that was clear.
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