James Young could easily be cut. That gets them down to 16. Then someone could get injured for the year. Or someone could get traded. If not he's in a competition with RJ H, Jordan M, Gerald G, and Demetrious J for the final spot. Maybe there's an agreement he won't sign with another team for at least a year. He's just not that far away from a roster spot
Your numbers are off.
Given to Make Team (barring trade) - 12 players
Thomas, Bradley, Crowder, Johnson, Horford, Rozier, Smart, Green, Brown, Olynyk, Zeller, Jerekbo
So 6 players for 3 spots
Bentil, Holland, Hunter, Jackson, Mickey, Young
But in reality Jackson and Mickey are almost assuredly going to make the 15 man roster (barring injury), so that really leaves just 1 spot for Bentil, Holland, Hunter, and Young. Holland seems the least likely and Hunter looks like the favorite for that last spot.