These are some of the most places that I have had luck finding three or more Dragon Balls per X....

Note: Not all the X's are completely accurate.

Click here to view Dragon Ball Hunting Area's.

I have heard that there is a better chance of a Dragon Ball drop in dungeons. Go to a dungeon with enemies around your level and kill mobs like a crazy person. Also monsters with Super or Ultra or Boss status have a higher chance of dropping a dragon ball as well.