Circle: has been used since prehistoric era and is regarded as the oldest ideogram. First it was founded in prehistoric caves to represent the sun or the moon.
An empty circle was used in order to indicate the eyes and the mouth.
In general, lots of concepts such as: infinity, protectiveness, unity, wholeness, the goddess, eternity, all possibilities, the endless and female power can be represented by circle. Moreover, it refers to human spirit and the inner individual.
However, the circles are not being used much in design.
Triangle: in ancient symbolism sometimes it refers to being well, good and healthy. Furthermore it is associated with holy and religious trinity especially in Christian symbolism and means safety as well as success and prosperity.
Square: Two-dimension shape represents a surface in both horizontal and vertical position and means ground, field, land or any elements of earth as well as earthbound matters.
Honesty, stability, equality, comfort, rigidity, uniformity and male are symbolized within squares.
In Buddhism the square located in the circle, indicates the relationship of the human and the divine.