Yes, I think this question is open to interpretation. Clearly in normal English one night is not cohabitation. But it is also clear that there are appeal judges who think that that the question "Have you and the sponsor ever cohabited?" should be answered 'yes' even if the cohabitation was for only a few weeks.
I think the key is to be very clear about your answer. If you say "yes" make sure the explanation clearly shows you were staying in a hotel together for 3 weeks and then 5 weeks. If you said "no", I wouldn't worry about it as long as any period of cohabitation was mentioned and proved elsewhere in your application (though if it goes to an appeal, maybe include an explanation of why you answered "no".)
For me, after reading those appeal decisions, I would answer "yes."