I understand that in "3-year period", "3-year" is hyphenated. But what about "3 year limitation period"? I once read that if the noun has more than one adjective, no hyphen is correct. Is this true?
No, you still need a hyphen there because 3-year is supposed to be one, semantically indivisible word. In this case, it's an adjective describing period and limitation period. Now, suppose that hyphenation was not necessary and that we had to make sure 3-year was still understood as one word, then we would have to write it like this: 3year. That doesn't really look like a normal English word, wouldn't you say?
If there is a space between 3 and year, then the two words 3 and year can be though of as though they are completely different words that are not connected with each other while they should be. In other words, you absolutely need to keep that hyphen there.