What is the ruling on renting an apartment to Christians (two women and a man) who do not know each other, and is the rental money permissible or forbidden?

It is not permissible to rent an apartment to a man and two women who are not his mahram, due to the assistance it provides for sin, their gathering, and their khalwa (private seclusion) in the apartment. If the landlord knows that the two women are not mahram to the man, it is not permissible for him to enable them to gather and seclude themselves in his dwelling. If he does, he must dispose of the rent money in charitable ways and for the benefit of Muslims.