The number of heart containers hints that the Lanayru Mining Facility is the third dungeon. There is some heart container discussion here: http://www.zeldadungeon.net/2011/11/japanese-skyward-sword-website-update/#comments
1 heart container seems to have been acquired from each boss preceding...
We can confirm that the first five dungeons are:
Skyview Temple Boss: Ghirahim
Earth Temple Boss: Scaldera
Lanayru Mining Facility Boss: Moldarach
Ancient Cistern Boss: Koloktos
Fire Sanctuary Boss: Ghirahim
Summing Up the New Skyward Sword Info in Quick Bursts
I am referring to this video: http://www.zeldadungeon.net/2011/09/nintendo-3ds-conference-skyward-sword-details/#comments
Slide down the dunes!
Water hose? Super Mario Sunshine, anyone?
This game is so different yet so familiar...