An anime of what may have transpired in Hell while Jesus was there for 3 days.

Already gets my vote. But rather than asking what happened in those 3 days, give us a clue as to what it is was that took place.

Good interesting concept that can go several different directions. And to bring out the right story across you need four things in the logline. Who's the Protagonist, Who/What is the antagonist, How strong are the stakes and what is the tone.
Good luck.

Intriguing idea! But who is the main character, what is his goal, and what are the obstacles in his way? Check this out for help: www.logline.it/howto/