budapest is so awesome!!! which hostel r u stayin at?
gooooo to serbia and stay at three black catz
(my friend mladen owns it). its only about 15 or 20 bucks one way from budapest, train goes from keleti every day at 13:00 and 23:00!
belgrade is sooo fun, trust me!!! everyone I tell to go there loves it! mladen gives you free home made serbian raijka, it has awesome nightlife, great people and sights!
I don't think I would try to do all those countries in three weeks. Travel time in the Balkans can be looong.
If you do head to Romania after but try to stay out of Bucharest. I didn't find it so bad but my friend has a hate on for it for some reason. Got hassled by a tranny prostitute I think. He's crazy. Its not the best but its not the worst.