What’s the Halacha regarding moving into a new house within these 3 weeks please?
Thank you for your question.
It depends what kind of house it is. If it’s your new home, then it is not the correct thing to do during the three weeks, unless there is a specific reason that you have to do it now. The reason being that it causes a lot of joy. It is for this reason that you would make a bracha when moving in to it. If it is not your own home, then there isn’t that much joy in it, and it is permitted.
Biur Halacha 223 D:H Bono, Kovetz Halachos ( Bein Hametzarim) 5-17.