Complex sentence: This third harmonic current (and indeed all harmonics divisible by three) are in phase on all the supply phases and will thus sum in the neutral to three times the individual phase (third harmonic) currents.
Split into simpler sentences: This third harmonic (and all harmonics divisible by three) current are in-phase on all the supply phases. These "triplen currents" sum in the neutral to three times the individual phase (third harmonic) currents.