B, only the valence of phosphorus increases in the reaction, from 0 to +5, so when 3mol is consumed in the reaction, the amount of substances that transfer electrons is 3mol×5= 15mol, so b is correct;

C, KClO3 undergoes a reduction reaction to generate KCl, which is a reduction product, so C is wrong;

D, P2O5 and water in the air combine to form H3PO4, which absorbs water in the air, and KCl is solid particles, so it should be white smoke, so D is wrong.

So choose B.