Newton's third law is valid in non-inertial frames when considering the forces between two objects in contact, as these forces remain equal and opposite regardless of the reference frame. However, the second law does not hold in non-inertial frames without introducing fictitious forces to account for apparent accelerations. This inconsistency can help determine whether a frame is inertial or not. The validity of Newton's third law can be used as a test for inertial frames, as interaction forces will always obey it. Thus, while the second law may require adjustments, the third law remains applicable in non-inertial contexts.