Tom:
Is the third message sent necessarily a 0? Or can the third message be a 1 if the original 1 message has been acknowledged before the original 0 message? -Matt & Grant

It can be tricky to discuss this problem because "third message sent" could be third "distinct" message sent or "third message sent including retransmission." I used letters in another answer to avoid this confusion. So, suppose you want to send packets A, B and C. A and B will be sent first. Then, if you recieve a prompt acknowledgement for A, C will be sent next (with the same sequence number as A). If you don't receive an acknowledgement in time, you will resend A instead (with the same sequence number used for A initially). So, the third message sent can either be the third distinct message sent or a repeat of the first.

Tom