The three adjectives describe a positive mood and correspond to the word happy. However, there are some differences:
Soy feliz = I'm happy (in general)
Estoy feliz = I'm happy (at this moment)Soy contento : INCORRECT
Estoy contento = I'm happy (pleased, content) with somethingSoy alegre = I'm happy (I'm a positive person, I can enjoy life)
Estoy alegre = I'm glad, excited (I'm in a mood for fun and laughing)
The opposite of ser feliz is ser desgraciado.
The opposite of estar feliz is estar deprimido or infeliz.
The opposite of alegre is triste.
The opposite of contento is triste or descontento.
The adjectives can be put in the following order according to their intensity: feliz >> alegre >> contento.
¡Hoy estoy feliz! = I'm happy today (Everything is perfect. Total and full satisfaction)
¡Hoy estoy alegre! = I am happy today (Not everything is perfect, but something very specific makes me smile)
¡Hoy estoy contento! = I'm happy today (I'm better than normal, I'm pleased, satisfied)
We can modify these adjectives with intensifiers (muy, bastante,...):
Me han regalado un libro y estoy bastante contento = They gave me a book and I'm quite happy
Me han regalado un coche y estoy muy contento = They gave me a car and I'm very happy
Me han regalado una casa y estoy loco de contento = They gave me a house and I couldn't be happier
The intensifiers totalmente, completamente are especially used with feliz:
SOUNDS GOOD: Estoy completamente feliz
DOESN'T SOUND GOOD: *Estoy completamente contenta
The adjective contento is only used with people. The adjective alegre can be used with objects (la casa, la ciudad, la noticia, el color, la música…). It means giving joy and good emotions:
CORRECT: El color de esta habitación es muy alegre = The color of this room is very cheerful (it gives joy)
INCORRECT: El color de esta habitación habitación es muy *feliz
INCORRECT: El color de esta habitación habitación es muy *contento
The adjective feliz can be used with places where people live and moments of our life:
Es un país feliz = It's a happy country
Ayer fue un día muy feliz = Yesterday was a very happy day
Esta película tiene un final feliz = This movie has a happy ending
The word feliz is used especially to congratulate people on some holidays:
¡Feliz Navidad! = Merry Christmas!
¡Felices fiestas! = Happy holidays!
Keep in mind:
contento / feliz + con + situación
Está feliz con su trabajo = He is happy with his workcontento / feliz + de + verbo
Estamos contentos de verte = We are glad to see youcontento / feliz + por + persona
alegre + nada
Estoy contenta por ti = I'm happy for you
Estoy alegre ____ = I'm glad