This video isn't in English, but it does a pretty good job in explaining the re-use:
https://youtu.be/I986q2nKeJQ?t=248 - Click for comparison video.
Pretty much everything is reused EXCEPT for the torso and the backpack weaponry...which explains why this figure released way earlier than its original release date.
All HT really needed to redesign was the torso. Everything else was a straight up reuse and repaint...regardless I'm looking forward to getting mine!
If you're talking about the underlying skeleton than I would agree that they probably based it on the Mark 3, but given that the chest, abdomen, shoulder armor, biceps, hips, thighs, knees, shins and calves are all redesigned, saying it's reusing most of the Mark 3 is incorrect. The head, hands and feet look the same, but all the parts between them are different!