Linked Accounts
To be able to gain triple experience, a veteran account must be linked with one or more new accounts. To create a link, the new account must be created using an invitation trial key sent from the veteran account. You can find the form used to generate these invitations by logging in to Battle.Net Account Management, selecting your "World of Warcraft" account and choosing the "Recruit a Friend" option from the "Referrals &Rewards" tab. The link between accounts lasts 90 days, starting from the day the new account was created.
Grouping and Range
To receive triple experience, a character on the veteran account must be grouped together with a character on the new account. The two characters must also be within a 100 yard range of each other to benefit.
Level Range
The game will only award triple experience to characters that are below level 80. Additionally, for both characters in the group to receive triple experience, the characters must be within four levels of each other; if they are not, only the lower-leveled character will receive triple experience.
Quest and Enemy Difficulty
Only quests and enemies that are of green or harder difficulty will award triple experience. Enemies that are of gray difficulty to characters will award no experience; quests that are of gray difficulty will only award a tiny amount of experience.