Turbin is a little intriguing- I'm not totally sure how he would fit with us, but there's talent there, even if he's never put it together. Maybe Belichick can figure out how to use it better than the Seahawks ever did. Probably more of a Blount backup than anything if he signs. He's intriguing in kinda the same way that James White is, though: in an entirely unproven way, that isn't really backed up by anything he's actually done on the field. Neither of them are Dion Lewis, who before breaking out spent the entire preseason demonstrating that he was had a unique skillset that nobody else on the roster could match.
As for Thomas... he used to be really good, but the fact that the 49ers cut him to sign Cadet seems telling. Maybe they're just idiots that didn't realize what they had, but it seems more likely to me that he's pretty much done.