Yes, both the Model S 75D and Model 3 75D will charge to 100% at the same time. The power rate they can take is identical.
Man a lot of speculation here with very little supporting evidence cited...
That's a good observation.
Model S 75kWh pack, AFAIK, has lower voltage (has less modules inside), as the pack was originally 85kWh (then 90, then 100).
Model 3 75kWh will have full house in the pack, therefore around 400V, Model 355 will be the stripped down version with lower voltage.
S75D and 375D will very likely not charge to 100% within the same time period. There are arguments in both directions:
Higher voltage on 3, therefore less amps required for the same kW rate compared to S.
Less cooling capability on 3, therefore slower in warm weather.
2170 might have better charge acceptance (voltage will hit the 4.2V mark per cell later than with 16850 cells), therefore tapering later.