At a guess from the dream you gave, it seems that your brain takes the same theme and runs through it three times, sort of like repeating an idea to yourself but with more detail on each run-through, as in each segment of the trilogy the same themes appear but with more and stronger symbols.

Seems to be just the way your subconcious thinks. Like, Hey, I have an idea! Then taking that idea and chewing it over and working it out a few more times. It's a valid way of tackling problems.

As for satan and satanic imagery, your subconscious pulls up symbols from waking life to get ideas across in dreams. Perhaps it's using imagery that you find frightening to try to scare you into getting a point across. You'd need to examine how you feel about the imagery to get meaning from it.

As for why the nightmares are getting worse...nightmares usually come from things that are bothering us while we're awake. So if this is a situation that hasn't been resolved, then it seems that your subconscious is going into overtime trying to make you aware of it so that you can address it. What issues are distressing you?