Actually. Not every shout is 3 words. Wanna know why? -WARNING SPOILER WARNING- In the dragonborn DLC. When you fight miraak he uses a FOUR word shout. I forgot what the words were but they translate into "I devour your soul". This is what he uses to instantly kill his dragon slaves. So no. But the shouts that the player gets yes they are ALL 3 words. Though shouts throughout lore and NPCS who can do shouts. No

Actually, not every shout is 3 words. There is a four-word shout in the Dragonborn DLC. When you

fight Miraak, he uses a FOUR word shout: ZII LOS DII DU. He uses this shout to instantly kill his dragon slaves.

However, this shout is not attainable by the player, so all shouts that the player can get have 3 words.