What's the difference between 기쁘다, 즐겁다, and 행복하다 in Korean?

Q. What's the difference between 기쁘다, 즐겁다, and 행복하다?

A.
   The definition in dictionary of 기쁘다 is "be satisfied and contented with fulfillment of one's desires. So, when we use '기쁘다' we specifically mention what we're pleasure about.
  For example,
 
               공부(를) 열심히 했는데 100점을 받아서 기뻐.
             

  On the other hand we don't have to be specific when we say the word '즐겁다' or '행복하다'.
  '즐겁다' means that be pleasure without unpleasant or any qualms. We use it like this.

               친구와 여행하는 것은 즐겁다.

  '행복하다' is having full satisfaction in one's life, and it can be used in widest sense. So, it can be used like this,

               결혼 생활이 행복해요.

  To sum up, '기쁘다' is felt in results, '즐겁다' is felt in the process, '행복하다' is felt in life.

  There are times when all three words can be changed.  However, '기쁘다', and '즐겁다' rarely change whereas '기쁘다' and '행복하다' are often interchangeable in Korean. Even then there are subtle difference.