The genitive of measure in English
Hello guys! I'm a bit confused with the genitive of measure.
How would you say: " This three-floor villa is surrounded by beautiful gardens and (...)" OR " This three floors villa is surrounded by gardens and (...)"
It is an example of the genitive of measure, right? I mean that kind of genitive which is used in expressions that indicate a measure (for example, distance or duration).

With countable nouns ("year", for hence) we use this scheme: a three-year period.
With uncountable nouns we use the noun in genitive+noun head, for example: two years' work experience.
With both count. and uncount. nouns we can use both schemes: a two-week/two weeks advance notice.
So, the noun in my phrase is countable. But it sounds strange to me to read "three-floor"...
Help me, ple-e-e-ase! :)

Question

The genitive of measure in English
Hello guys! I'm a bit confused with the genitive of measure. 
How would you say: " This three-floor villa is surrounded by beautiful gardens and (...)" OR " This three floors villa is surrounded by gardens and (...)"
It is an example of the genitive of measure, right? I mean that kind of genitive which is used in expressions that indicate a measure (for example, distance or duration).

With countable nouns ("year", for hence) we use this scheme: a three-year period.
With uncountable nouns we use the noun in genitive+noun head, for example: two years' work experience.
With both count. and uncount. nouns we can use both schemes: a two-week/two weeks advance notice.
So, the noun in my phrase is countable. But it sounds strange to me to read "three-floor"...
Help me, ple-e-e-ase! :)

Answer

You've made a very simple rule seem very complex! :)

Here's the simple rule: adjectives don't change.  If a noun works as an adjective, then it usually stays in its original form.

If you're counting the noun but you still use it as an adjective (three-floor villa, a thirty-year-old woman) then the noun doesn't change.