The Rach 2 was originally more popular than the Rach 3, and except when related to Shine, I still find it's more referenced.
The third piano concerto has the reputation of being the most difficult piano concerto, and it's pretty high up there. I haven't attempted it myself, but I've seen it played.
Rachmaninoff's birthday was April 1st ... April Fool's Day. Neat fact.