If instead of molecule, we assume a molecular of a monatomic gas, which having of a one atom, the translational motion can take place in any direction in space. Thus it can be solved along three co-ordinate axes and can have three independent motions and as three degrees of freedom. A mono atomic particle can also rotate but due to its small moment of inertia rotational kinetic energy is not significant. Thus it does not check rotational degrees of freedom.