Three phase circuits have problems with triplen harmonics (the third harmonic and its odd multiples) because they do not balance on the neutral; instead they add up. All other harmonics will balance on the neutral if they are from a balanced source (eg. the 5th harmonic from a three phase rectifier).
The neutral in a residential single phase multiwire circuit does not have a problem with triplen harmonics; they balance just like any odd order harmonic. However in this case the neutral will have problems with even order harmonics.
With rectifier power supplies, even order harmonics are much lower intensity than odd order harmonics. You only see even order harmonics from the hysteresis of the load.
-Jon
