Which is the odd one out of triangle square or hexagon?

Triangle: The other shapes have an even number of sides.

Square: The numbers of sides on the other two is divisible by 3

Hexagon: For sides of a fixed length, the other two shapes are rigid structures (the square can distort to a rhombus but then it is no longer a square.)

or

Triangle: It can have all acute angles.

Square: It can (must ) have all right angles.

Hexagon: It can have all obtuse angles.