I played the second almost 10 years ago for my graduation concert (accompanied by a second piano, I wasn't lucky to play with conservatoire orchestra

) and that was a really exhausting experience. Rachmaninoff was a first class virtuoso and that concerto is one of the most uncomfortable works I've never played. I also started the third concerto but never completed it, too much dense, too large and maybe too extreme virtuosistic, too restless (45 minutes of jumps, huge chords, double notes, OMG!) for my skills (and patience). I have also the scores of the other two but I have not even read the score at the piano but at sight (without playing it, only viewing the score) both looks also terribly difficult. Maybe the first one has a more conventional writing but that doesn't mean it has to be easier...
So, in brief and IMHO, I cannot order them. I think that every one of them is a top piece in the repertoire, incredibly difficult to play and has its own different but always challenging difficulties.