First of all, from the convenience of life, parents living on the third floor may face the inconvenience of going up and down the stairs. If they are old or unwell, it may bring trouble to their daily life. Daughter-in-law lives on the first floor, far from her parents, which may affect the interaction and care between family members.
Secondly, from the perspective of Feng Shui, if the living status of the younger generation is higher than that of the older generation, it may be regarded as disrespect for the older generation, which is contrary to the concept of "the elder comes first" in China traditional culture. But this statement has no scientific basis and does not apply to everyone.
Generally speaking, it is not absolutely bad for parents to live on the third floor and daughter-in-law to live on the first floor. It mainly depends on personal life needs and concepts. If possible, consider setting up a living area on the same floor to facilitate the interaction and care between family members. If conditions do not permit, we can also improve the quality of life of our families in other ways, such as installing elevators.