Running single-phase through two or three windings of a three-phase inductor; why does the direction of the middle winding matter?
Because the 3 windings are on the same core they have mutual inductance. When hooked up in one direction the mutual inductance of coil B adds to the others, but in the other direction it subtracts from them. It acts like the secondary winding of a transformer, with the voltage induced into it either adding to the primary voltage or subtracting from it. 

If coil A and B were both on the same leg of the core then the coupling between them would be close to 100%, so when connected in antiphase their inductances would almost completely cancel out. This configuration is sometimes called a 'non-inductive winding'.  

However with the coils on different legs only half the flux induced into the core by coil A goes through B (the other half goes through C) so the coupling to it is only 50%. With half of B's inductance cancelled out it only increases the total by 25%, whereas when connected in phase it adds all of its inductance (giving the expected 50% increase).