Yes, it's much faster than levelling a single character. See this link on group xp per mob kill: http://www.wowwiki.com/Formulas:Mob_XP#Group_Experience. If I'm reading it correctly, a group with equal level characters would normally get half of the xp per kill vs a solo kill. However, multiply this by 3 and each character is getting 1.5 times the xp of a solo, non-RAF kill.

If you're questing, you'll also get 3 times the xp per quest, and quest xp is not affected by the size of the group.