How to use shout power levels

Yes, all shouts involve 3 words.

Here is releveant information on the words:

You can learn thu'um words even if you dont have dragon souls.
However dragonsouls are needed to activate a shout word.

— Wikia - Elder Scrolls V: Skyrim - Word Wall

The player can choose to only vocalize part of a dragon shout which will lower the recharge time of the shout but cause the shout to be weaker. In an interview Todd Howard said every shout has 3 words and there is 1 word per word wall.

— Wikia - Elder Scrolls V: Skyrim - Dragon Shouts

So, there are three states for a word:

• Invisible: The word hasn't been learned nor activated.

• Grayed out: The word has been learned, but not activated yet.

• White: The word has been learned and been activated.

Here is a full list of the 24 dragon scouts:

• FUS RO DAH: Force Balance Push (Unrelenting Force)

• IIZ SLEN NUS: Ice Flesh Statue (Ice Form)

• STRUN BAH QO: Storm Wrath Lightning (Storm Call)

• YOL TOOR SHUL: Fire Inferno Sun (Fire Breath)

• TIID KLO UL: Time Sand Eternity (Slow Time)

• WULD NAH KEST: Whilwind, Fury, Tempest (Whilwind Sprint)

• RAAN MIR TAH: Animal, Allegiance, Pack (Animal Allegiance)

• SU GRAH DUN: Air, Battle, Grace (Elemental Fury)

• FIN ZI GRON: Fade, Sprint, Blind (Become Ethereal)

• FO KRAH DIIN: Frost Cold Freeze (Frost Breath)

• ZUL MEY GUT: Voice Fool Far (Throw Voice)

• ZUN HAAL VIIK: Weapon, Hand, Defeat (Disarm)

• LOK VAR KOOR: Sky Spring Summer (Clear Skies)

• HUN KAAR ZOOL: Hero Champion Legend (Call of Valor)

• JOOR ZAH FRUL: Mortal Finite Temporary (Dragonrend)

• KRII LUN AUS: Kill, Leech, Suffer (Marked for Death)

• LAAS YA NIR: Life, Seek, Hunt (Aura Whisper)

• OD AH VIING: Snow Hunter Wing (Call Dragon)

• KAAN DREM OV: Kyne, Peace, Trust (Kyne's Peace)

• FAAS RU MAAR: Fear Run Terror (Dismaying Shout)

• SLEN TIID VO: Flesh, Time, Undo (Ressurect Dead Dragon, not available to player, Alduin)

• N/A: N/A (Meteor Swarm, not available to player, Alduin)

• FIIK, LO, SAH: Mirror, Deceive, Phantom (N/A, not available to player, Greybeards)

• VEN, MUL, RIIK: Wind, Strong, Gale (N/A, not available to player, Alduin)

— See Wikia - Elder Scrolls V: Skyrim - Dragon Shouts for more details.

You can use WASD to navigate through the constellations for each skill. The perk trees are accessed by zooming in to the constellation using W or by clicking.