The answer to this can be found in Chapter 2. This is where Francois put Buck when Buck was still very new to being a sled dog. The reason Francois put Buck there was so that the other two dogs could teach him how to do his job correctly.
Dave and Sol-leks are said to be particularly good teachers. They knew what to do and they would not tolerate it for long if Buck made mistakes. An example is given in which they fight him when he gets the traces all tangled up. By being between them, Buck learns how to be a sled dog fairly quickly.