The reason for the response of 3 and 1 is because the 2nd due or first due truck is not in service. So by FDNY rules they could go down to 1 truck. However the BC can then request the 2nd truck who would be dispatched in any order, chould be the 4th due on the 2nd but because all the trucks between him and the 1st are not in service he would go.
An Example of this would be one of the 10-77's on Central Park West.
Due to a 2nd Alarm at Mt. Sini Hospital the ticket went out to
Engine 65 - a Queens Engine and Ladder 16. The dispatcher has to give two and one unless in fallback step 3, then 1 and 1.