TrainingPeaks is offering up stage data files on riders they work with.
Just go here: https://www.trainingpeaks.com/tour-de-france/
(you do need to share an email address for access)
For the Stage 3 TTT, Matt Hayman had a normalized power of 348w, which included a half-hour of cool-down. Take that out, and it’s easy to see his average power up near 400w.