Which of the following statements is correct?

1. Every rhombus is a square.

2. Every parallelogram is a rectangle.

3. Every square is a rhombus.

4. Every rectangle is a square.

1. Every rhombus is a square — False (a rhombus has all sides equal but not necessarily right angles).

2. Every parallelogram is a rectangle — False (a parallelogram does not necessarily have right angles).

3. Every square is a rhombus — True (a square has all four sides equal, which is the defining property of a rhombus).

4. Every rectangle is a square — False (a rectangle has opposite sides equal but not necessarily all sides equal).

Hence, option 3 is the correct option.