For me I think 2 bores is standard, I have 2.8mm OD, 3mm OD, 2mm OD tubes. Drilling the holes is a tricky task. I broke multiple shells trying to fit 2 bores. The best way is to use a small
diamond burr preferably 1/1.5 mm Diameter and slowly enlarge the hole with the same bit. Another way would be to drill a big hole and fill it wit the tube and resin for a cure.
I think the 3 bores is usually made with at least 2X1.5mm OD tubes and one 2mm OD. but I am not sure. I also think a 3 bore design might be easier to achieve using the filling method rather than drilling.
I don't think it uses inductors, just the regular RC high pass filter with cap and resistor only 4th order.
also If I am not mistaken 4th order will have a 180deg phase shift at the crossover point thus the need to correct with the phasefreq (I think it is just longer tube to induce delay to the sound to correct phase, a thing I have discussed earlier in this thread without knowing it might be the thing behind freqphase) or maybe invert polarity of the drivers.