You can only get the third BP at the gate or ticket counter in that city. They issue BPs at the gate, so you would have to get it there. Keep in mind that only a tiny portion of our Customers travel with double connections. And of those that do, many are on point to point fares. For instance they are traveling from A to D and change planes in B and C. They may have bought one ticket from A to C via B, and a separate ticket between C and D. If you book these two tickets on separate reservations, then you can check in online for all three flights.