beat all the boses in the light realm

To get all 3 of the medallions, you need the magic mirror, access to the dark world, the
book of medura, the master sword, and the magic flute would help greatly.

Ether medallions
If you have the flute, fly to the mountain area. Trek up to the place where you fought
the last boss in the light world (aka the giant tower) dont enter, but go to the left of
the tower and walk up. You should see a bridge to your left. Go across the bridge,
select the book of medura, and press the A button in front of the green stone tablet. You
will get the ether medallion, which freezes enemies. Watch your magic meter!

Bombos Medallion
In the dark world, head to where the desert was in the light world. Linger around at the
entrance to the desert in the dark world until you find a small fence of pegs. The pegs
are widespread, so you don't need the magic hammer for them. Walk into the space and use
the magic mirror to teleport into the light world. DO NOT JUMP OFF THE CLIFF! Next,
walk to your left and open your book of medura and read the stone tablet. You will now
get the bombos medallios, which burns your enemies. Watch you magic meter!

Quake medallion
Head over to the entrance of Zora Falls in the dark world. Do not enter. You should see
a circle of rocks in the water to your left if you are still on land. Read the nearby
sign. It reads: DO NOT THROW ANYTHING INTO THE CIRCLE OF ROCKS or something like that.
Anyways, pick up the sign and chuck it into the circle of rocks. If you miss, use a bush
or a rock. When you make it in, a giant fish comes out and gives you the quake medallion
if you stop bothering him. Bravo, you have all three medallions, the last of which
summons an earthquake to shatter all foes. Watch your magic meter!

*** The medallions do not work on certain foes or bosses.