mg: Your instincts appear to be serving you just fine.
Third-floor and easily-remembered both conform to a rule I learned long ago: to hyphenate words that are strung together to form adjectives. Aside from that there are obvious situations, e.g. the McCain-Feingold bill, that would retain the hyphen even if the noun bill were not included. Thus if I go to high school, I am a high-school student.

Now I should look into all those commas I use.