First pitcher you can tell he's stepping across the midline (or across his body), he's blocking his hips a bit so he's not getting the full rotational benefit he'd have if that was fixed. This is also causing him to fall off to his right a bit (can see it a lot, but especially around 1:41), so that energy that could be going forward is being wasted (meaning less velocity etc...).
Also it appears that he's getting his throwing arm up a bit too soon before his front foot is landing... that's a timing issue that's costing in velocity and may give him some arm troubles in the future. I think this is more of a symptom than the cause which is it looks like he's reaching out with his foot instead of just driving his body towards the plate, which is in essence slowing him a bit - which means his arm is getting to its cocked position too soon. If you pause the video at 2:19 it basically shows this perfectly.
Also, with the slow leading with the foot instead of driving his body, he's not getting a good bracing action with his front leg and he winds up throwing off one leg here (can hurt velocity and accuracy as it's not a stable of a base than two legs).
Now moving on to the second pitcher, I'm not a fan of him moving so slow at the start, but he does move pretty quickly out of it and gets decent trunk flexion. The only problem is again, he's landing way across his body
As you can see, he winds up blocking his body off, and in order to compensate for him going so far across, has him moving towards first-base side. So again, instead of these forces going towards home plate, it's being wasted. Just look at the picture though, you'd think he'd be more set up to throw into the first-base side dugout rather than at the catcher. You don't get as stable, velocity and accuracy both diminish from what it could be.
Third dude looks to have some early hip rotation (can be seen clearly from 10:13-10:15) and not enough drive (he's taller than the other two but getting to about the same landing spot maybe less? Could be really tight, may need some flexibility stuff, or he may need a hot poker to his ass to get him to move a bit quicker [although on that mound he'd probably overshoot it so you're limited by the equipment you have) means this kid is all arming this thing + because he's not getting that whole drive into it means no flat back finish. Flat back finish allows for a little bit of velocity but it also allows the arm a greater arc of deceleration to disperse forces, which in theory should result in lower injuries.
Those are my thoughts. Best of luck!