If the meter is a Fluke 33 as described here:
http://www.metrictest.com/product_info.jsp?mfgmdl=FLU%2033 then it should be _perfect_ for determining if harmonics are the problem. Apparently the device can measure true RMS current up to 10000 Hz, and can measure crest factor, which is a measure of harmonic distortion. I've never used one of these myself, but take a look at the datasheet on the page mentioned above.
Get measurements of the crest factor on each phase, and the neutral current, and that should tell you the story.
THD is total harmonic distortion. I doubt that fittings have this marked. Some devices might have power factor and crest factor marked.
_Any_ 'non-linear' load will have a certain amount of harmonic current flow. Computer switching power supplies are one example, because they have input rectifiers that only conduct at the peak of the AC waveform. But transformers are also non-linear, especially when close to saturation. Arc lamps are also non-linear, because the voltage has to get high enough each cycle before they start conducting.
Triplen harmonics are harmonics which are third multiples of the fundamental. In three phase systems, triplen harmonics flow through the neutral, since they are not balanced phase to phase.
I don't really have a list of web sites that would help you; I had to read up on this stuff about a year back, but all I could do now is more google searching. If you search on the term 'IEEE 519' and look for 'tutorial', you'll probably find some good stuff.
-Jon