Ahh, yes I see why I was so confused I was determining the impedance of the single phase transformers by mistakenly treating them as three phase instead of single phase, thus when I tried to use that impedance on the three phase system I was coming out much higher than I felt it should. Always...
If I have 3-single phase 75kVA transformer configured into a 3-phase bank does the individual fault current of each 75kVA transformer add up for the total fault contribution on the three phase system? (i.e. each 75kVA contributes 5kA, then 3-phase system would have 15kA)