Upon turning the time back 3 days for the first time with your ocarina, talk to the Happy Mask Salesman and he will teach you the song of healing and transform your Deku spirit into mask form. Using the c buttons to equip it, it will let you transform into your Deku form. Limitations of this mask is that guards in clock town will think you are too young to leave the gate and dogs will jump you. However, it will let you shoot out from Deku flowers to reach high platforms and ledges, get into the Deku Palace and shoot bubbles.
After you receive the Lens of Truth and climb up the side of the mountain, you will meet the Spirit of Darmani inside of a cave. Play the Song of Healing to allow his soul to rest, and you will obtain the Goron Mask. This mask will transform you into a Goron and allow you to roll up into a ball and move at fast speeds, also allowing you to ramp off of ledges and break giant snow mounds. However, since you are now a muscular and heavy weighted body, you will sink in water.
Upon finding Epona again, jump over the wall to gain access to the Great Bay. Floating in the water will be a nearly dead Mikau, a member of a local Zora band. After explaining the situation to you, play the Song of Healing and you will get the Zora Mask. It will allow you to become a Zora and swim and dive at fast speeds, even against rough currents. You will also gain the ability to shoot fins at enemies and use a shield that can damage enemies that touch you.
Fierce Deity’s Mask
After you have gained every mask in the game, defeat Skull Kid on Clock Tower and pursue him onto the Moon. Upon arriving, approach the boys running around and give them all of your masks. Then the child wearing the Majora’s Mask will feel sorry for you because you do not own any masks and he will give you the Fierce Deity’s Mask. This will transform you into a powerful warrior with a huge, two-handed sword. While extremely powerful, it can only be used in boss rooms.