The gear ratios are the same in all 3 R33 GTRS and 5th is O/d on all, series III has improved synchros though.
Series III is the only one with red inserts in the seats ( others are blue). S III has bigger lower lip , rear foglight, an opening( cutout) on the passengers side front blinker, xenon h/lights that have a different top bracket to the other 2, GTR on the fuse cover in the engine bay and some s III have factory leather trim as well.
Series I cars have one airbag standard but some will have 2 ( was a factory option). Series I have different steering wheel to the other 2 and different dash suround ( flat ( not bubbly). Some series III had momo steering wheels .
I'm sure there are other differences that don't come to mind right now .
By the way none of them weigh anywhere near 1750 kg, depending on the spec GTR , they are all about 1500-1540 kg