Out of the games I have played, a 3 heart challenge on Twilight Princess would be the easiest, as I have done it several times and not died once. I guess they made that game too easy, or in the case of the fight against Ganondorf, hard enough to kill you but they over compensated by littering the arena with skulls that you could break open to find hearts. Ocarina of Time would probably be the next hardest up from Twilight Princess, as to be quite honest, I find that game quite easy on the 3 heart challenge to (although, I probably would die a few times).
Majora's Mask would probably be the second hardest out of the games that I would bother to attempt a three heart challenge on, as I find the bosses in that game to be quite hard, and the game to be harder than both Ocarina of Time and Twilight Princess overall (probably combined, to). I generally lose more than 3 hearts against each boss from the Snowhead Temple onwards.
So I'd definately have to say the winner would be the original Legend of Zelda by far. I get killed by some of the stronger normal enemies half the time, and that's with all of the upgrades. I certainly wouldn't be able to beat Ganon with only 3 hearts, I probably couldn't beat most of the other bosses with 3 hearts.
