I think the film in the three-strip Technicolor cameras was Super-XX. But as you see in the Technicolor page on Wikipedia, with all the light losses in filters and beam-splitters, the effective film speed of the camera was ASA 5. It took a lot of light.

They could obviously make whatever masking they wanted to when making the matrices from the camera negative. They also controlled exposure, called "timing".

There was a "colorist" on the set of three-strip Technicolor movies who made sure that the colors were "in gamut".

As for contrast build-up problems, the folks who light movies control contrast ratios very carefully. Nothing is shot in natural light. So they can cope with film processes which have very high contrast by dialing down the lighting contrast ratio.