On my first attempt, I rooted Finnelzi and Tehana, then snared Rendolf and kited him away from his companions. I killed him in a couple of minutes, but the other two despawned (this was well before 1:00 AM game time). I looted Rendolf's crystal, and tried to restart the event several times, but they did not spawn on subsequent midnights.
I guessed that they did not respawn because I already had one crystal in my backpack so I crossed my fingers and destroyed it. They spawned normally the next midnight.
Snared all three and kited them around as a group. Each then dropped his/her crystal as expected.