Stauskas could be completely fresh, while Marcus Smart and Pressey may have already been through several sets of workouts.
Stauskus also dips down before shooting.
As long as pressey can make the open 3, he will be ok.
Smart shot looks better. As long as he doesnt wind up too much or turn sideways after the follow through(in the workouts he has got this fixed) he will be good
Dipping the ball isn't unusual - most shooters dip it some so they have more travel as they push the ball up.
Where Smart's mechanics would break down (in actual game shots the last two seasons) is he would lean forward, bent at the waist and have his arms out on the dip. This would start the ball down between his knees. This would mean he would be first pulling the ball in to him (with the ball often ending up cocked above & behind his head) and then pushing it back out on the actual shot, while his back would be straightening up, pulling off the shot. The result was often a flat shot. The problem is exaggerated on 3s because he's trying to muscle it for the extra distance. It also exposes the ball more to defenders.
You won't see this problem in practice shot vids like posted here, where they are basically taking upright set shots with the most minimal jump.
The starting place to solve this is in the knees - bend 'em son. Don't bend at the waist. Watch a thousand hours of Ray Allen vids (or just review this one game here a zillion times) and look at his legs. Do that.
https://www.youtube.com/watch?v=poVuS0PAuu8The worry is that Smart's been doing that bend over so long that it will be hard to fix. Stopping the lean and using his legs right will be a start. Beyond that, he's got to stop pulling his off shoulder away as he lets it fly. That's rotating him off of square. That's going to be really hard to fix.
He's a hard working kid and he's going to have great coaches working with him. Hopefully he will work this out. But don't expect massive improvements right away.
Ultimately, if he never fixes it, I personally don't think it decreases his value much. His ability to explode to the hoop and draw fouls is a powerful offensive weapon in itself. Last year, only Green & Bass ever seemed able to get to the FT line (Kelly showed flashes in those late season games). We need more of that ability from the guards & wings.