3x experience gain seems to be based on "LINKED" accounts, not friended accounts. You MUST have an old account that invites another person to play. That new person (YOU in this case) then makes a new account using the new CD Key from the e-mail. The way the accounts get linked is through this CD Key. The new account and your original account now get 3x experience for 90 days as long as they are playing in a party together. Only characters at the same level and under 60 will get the exp. You can have as many characters getting 3x experience as you want as long as they are partied with a linked account.

I repeat, you must have linked the accounts by starting a new account using the trial CD generated by an older account.

If you already had 5 accounts before this offer, I see no way to get the exp bonus without making even more.

If every character in your party is linked to another character in your party, it should work out.