Not 3 times, but yes, I've had this as well.
It's very annoying, specialy when you end up dead because of it and trying to explain your party the thing is bugged sounds like you are making excusses.
A very very annoying bug for a very annoying game mechanic.
the lancer boss was bugged as heck last night. ive seen the girl occasionally reset in the fire room, but 17 times :-/ (almost always when she was below 30% hp). the war had to get it down to like 80% then run around the room getting all the fireballs together for one epic hit (since you cant 100% her, as the fireballs dont last long enough).
we just "burned" through the second one. not fun on HM :-/
Now that's the kind of bug I'd love to get.
The only one I've experienced has been the snake head not spawning, period.
you can also steal agro and make the boss forget what she's doing. This will result in someone being pulled in but her not actually doing the attack (so someone is trapped).
skipping over a cycle ive had cause one snake to despawn before he turned stone and another never spawn