I definitely feel McCarthy is the best of the bunch. He does indeed have the best arm and he puts some good touch on his deep balls. His release is slower and needs to come out with a little bit more conviction, but the upside is there for him to become really good. He could really stretch the field with his mobility and deep accuracy. He's got a little bit of Jack Miller in his game.
While I believe McCarthy is the best of the group, I also quite like Kyle McCord. His arm isn't nearly as strong, but it's a little less "cumbersome", if you will. McCord has very impressive accuracy and touch in the intermediate parts of the field. McCord is definitely the more "pro-ready" QB, if you want to apply that to high school sophomores. His technique is smooth; his footwork is good (better than McCarthy), but still could use some work. He definitely has to overcompensate a little more on deeper throws, but his accuracy makes up for it, at least at the high school level. McCord is more in the mold of Nick Foles, but more mobile.
Preston Stone is a very mobile QB, but with not as strong of an arm (I'd put it somewhere between the first two guys). He'is quick to get out the pocket, though. He's got higher upside than the other 2, mainly because some of his throws are absolutely beautiful, but his passes have a lot higher variation, as on certain throws, the ball wobbles, pointing to his weaker velocity. I also see too high a number of desperation throws, hoping the WR comes down with it (where the DB never looks back those plays), Trace McSorley-esque. That, though, is an unfair comparison as he's closer to Shea Patterson than McSorley. All 3 guys are going to be elite guys come their senior years.