This is essentially, fly 2 get 3rd flight free promotion.
Don't forget to register for the promotion.
As an American Express® Cardmember, receive $25 off plus 50 TrueBlue points when you book a JetBlue flight with any American Express Card. Since you can earn a free roundtrip flight anywhere JetBlue flies with just 100 points, this exclusive offer gets you halfway there! If you're not already a member, just sign up for TrueBlue, JetBlue's customer loyalty program, then purchase a roundtrip flight with any American Express Card by May 13, 2008, for travel between May 1 and June 18, 2008. If you have already registered, please go directly to jetblue.com/springdm to book your flight.