Don't tackle the Rach 3 now, trust me, you'll be happy you waited until later on in life.
Check out Gershwin f minor, Mendelssohn g minor (classic), Mendelssohn D major, Brahms concertos no 1 and 2 (these are incredibly difficult though, like the Rach 3, but if you are looking at the Rach 3, i'd recommend checking these out), All of the Prokofiev concertos, Chopin concertos 1 + 2, Liszt concertos (you can learn a full concerto, they are relatively short), Schumann concerto in A minor, Scriabin concerto in f sharp minor, and Khatchaturjan concerto.
Before selecting a concerto, listen to ALL of them. Remember, you will be spending a lot of time on these.
If you are going to do only one movement of a really hard concerto, you might as well learn the Rachmaninoff Pagannini variations, they are quite difficult, but 25 minutes long.
The Liszt Totentanz, Stravinsky Burleske, and Franck Symphonic Variations are all beautiful, and amazing in their own way. These are all one movement, and somewhere from 10-18 minutes long. All of them are challenging as well. You must give these a listen.
If you need ANY of these concertos, I can send them all to you through Aol Instant Messenger.
If you don't want me to send you any of them,
www.karadar.com is a good start. Go to that link, click on Cocoa, sign up, make a compilation, and download.
Unfortunately, the recordings aren't the best, but you'll get the idea.