"Three page book" or "Three pages book"
"Three pages book" is always incorrect. "Three page" is an adjective phrase that describes the book. In English, adjectives are not agreed in number with the nouns they modify.
"three year old" or "three years old"
Both may be correct depending on context. "The boy Simon was three years old." "Simon was a three year old boy."
"three hour journey" or "three hours journey" or "three hour's journey".
Three hour's is wrong because the hours do not possess the journey. The apostrophe in "hour's" indicates possession. Otherwise the answer is the same as in your first example. "Three hour journey" is right.
Student or Learner