The third movement is just hell, put plainly. Literally, if you've played the third movement, you've been through hell and back. It's by far more difficult than either the first or second movements. Believe it or not, it's not technically impossible. In fact, it is very playable. The problem is that so many people perform it but can't actually PLAY it, if you know what I mean. Some performances have perfect notes and rhythms but are so boring or awful-sounding that they make me want to put a bullet through my head. Also, the voicing is just plain ridiculous. There are places where there are three or four voices voices in the right hand, and no one short of Ashkenazy, Kissin, Orozco, Bronfman or Rachmaninov himself can play those lines beautifully.
As for the other movements, the first movement is probably the most playable (and the easiest to sound good on), as I played it but still can't play the other movements. The second is more difficult, but it's definitely playable. You just need really good voicing and a very "quicksilver" touch, especially in the faster waltz section (F# minor).
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.