Tip: You can easily level your first class to 50 by doing nothing but the main storyline, class quests, Hunting Log and any FATEs you come across. (In the late 40s, you may have to add a few leves and extra FATEs, the game slows to a crawl around 45-50). You can safely leave all side quests untouched to come back to when you level your next class. This will make leveling your second and third class a bit easier.
Tip 2: If you know you will level all classes, don't be afraid to greed roll every pink and green (and eventually blue) gear you come across. You will use it later, and you are not taking away anything from people who need that gear right now, because Need rolls have priority over Greed rolls.