three months' holiday 可以表达成three-month's holiday吗?为什么在这还要" '

three months' holiday 可以表达成three-month's holiday吗?为什么在这还要" 's"?
three-month-holiday,如此表达行不行,就好像ten-year-old 一样?
three-month's holiday 有了“-”在中间为什么还要" 's