"They are not doctors."
Actually no. If ALL the doctors referred to in the sentence are female, your sentence would be right. However, same as it happens in Spanish, if there's at least one male doctor in the group, the character used in the sentence is the same used for male in plural. So, 她们 is used if ALL the doctors are female, 他们 is used if all doctors are male AND if there's at least one male doctor in the group.