In a regular octahedral molecule, $\mathrm{MX}_6$ the number of $\mathrm{X}-\mathrm{M}-\mathrm{X}$ bonds at $180^{\circ}$ is:

In octahedral structure $\mathrm{MX}_6$, the six hybrid orbitals $\left(s p^3 d^2\right)$ are directed towards the corners of a regular octahedral with an angle of $90^{\circ}$. According to following structure of $\mathrm{MX}_6$, the number of $\mathrm{X}-\mathrm{M}-\mathrm{X}$ bonds at $180^{\circ}$ must be three.
Related Theory In octahedral, two orbitals contain lone pairs of electrons on opposite sides of the central atom. The remaining four atoms connected to the central atom give the molecule a square planar shape.