Sally lives in a three-bedrooms house but Amy lives in a four-bedroom house.
> The word "three-bedroom" is an adjective describing the house; therefore, it should be in it's singular form. Let's use "but" to make an independent clause.

And Sally's townhouse has two floors, but while Amy's apartment has one floor.
> We can't use the possessive form of "Sally's" if we don't have a subject to support the idea.