I enjoy doing the collection quests in a sort of masochists sort of way. It's a way to grind up a little bit of xp you lack from always being in a group.
But compared to other multiboxers, I tend to skip doing instances more than once unless I'm leveling 4 or less characters.

Since you're leveling 3 characters, you could always use the Athene's method of leveling. But this requires a prot pally and a mage friend willing to help you out.
I really hate this guy's attitude but the leveling method is solid. It uses the tap system bug.

Also mods, from what I've seen and heard this method does not break any rules, however it is a bit on the iffy side of taking advantage of game mechanics bordering exploitation. If any mods deems this as against the rules, please feel free to remove it.

http://www.youtube.com/watch?v=xj6OkLKCecI