As a teacher; let's force student to do things he doesn't want to. Not to help him learn. But to spite him and steal his techniques. Yeah. Fail teacher.
If it was me, I would break those three at this point. If you give up, they won't. If you win, they will probably still plot against you. So forcing them to bow out is best. No need to kill them. Just make them unable to come to classes for 3 months.