This is not a walkthrough. This will merely describe the general course of action you must take to unlock each ending. Spoilers.
Ending A
You can achieve this ending by activating the three telectroscopes throughout the map in order to open the bunker found under the museum.
This ending involves fighting three different bosses back to back. An AoE attack is recommended for the last one.
This ending involves fighting three different bosses back to back. An AoE attack is recommended for the last one.
Beat them all, and you’ll get ending A.
Ending B
This is one of the simplest endings in the game. Kill everyone, go to the tower in the middle of Prehevil and ascend its lofty heights. Tell the man at the top you want to kill him.
Kill him.
That’s ending B.
Ending C
You get ending C by killing everyone like in ending B, but before day 3 morning. You have to tell the final boss that you would like to join him as well.