After realizing the effect of strain on structural properties, it is desirable to test the monolayer's mechanical strength and stability for their applications in nanomechanics. Considering this fact, we have made a comparative investigation of mechanical properties for all three monolayers. For 2D hexagonal symmetry, it is sufficient to find two main parameters such as Young's modulus and Poisson's ratio. The in-plane stiffness (C) is used as an alternative to Young's modulus because of reduced dimensionality. It measures the rigidity or flexibility of the material and it is defined as