When the size and shape of a macroscopic object matters, it can often be approximated by a "rigid body". A rigid body is a system of particles in which every pair of particles has fixed relative displacement. This is an approximation because the smallest parts of objects are atoms which do not have definite positions according to quantum theory. It is also an approximation because a change in position of one particle cannot affect the position of another particle instantaneously according to the theory of relativity.
A rigid triatomic molecule, which can translate and rotate but not vibrate so it has 6 degrees of freedom, 3 translational and 3 rotational but no vibrational.