I've heard the third is the killer. My current teacher said he was asked to perform it in three weeks... He hadn't ever played it. He said he learned the first movement and it was "kind of easy." He learned the second movement and it was "kind of easy." When he got to the third movement, he said "To hell with this." (He finished it, just not in three weeks, lol!)My former piano teacher said when HE learned it, after the first page it was just one technical difficulty after another...But I think the third is considered the hardest of the three. Not sure about the first two (which one is easier).
just for the record, rachmaninoff wrote four piano concertos. but yes i would agree that the third is the hardest of the four.
Also, it sucks when I'm listening to the second movement on YouTube, and you know how it ends in a musical cliffhangar? (it ends on the 5-chord and resolves into the first chord of the 3rd) Well, the movements were all in separate videos, and the internet crashed before the third mvt loaded, so I couldn't hear the chord resolve! I was so miserable and I couldn't eat drink or sleep for 6 hours until the internet launched up again.