I've read about there being a 100,000 pt secret hidden in level 3, but so far I can't figure it out. how do you achieve it?
Where did you read that?
As far as I know, that's completely false.
2 people mentioned it on a previous thread. it's supposed to be a parallel dimension
They mean that when you go through the door that opens when you complete the level, it's possible to get 100k or more in the parellel levels. No big secret or anything, just entering the parallel level.
Getting the Gordon key in 3.0 gives you 10k. Killing the enemies in order in 3.1 and 3.2 gives you 2x40k cristal. Killing the enemies in order in 3.3 gives you 10k cristal +3x4,5k cristals which equals 113,5k without counting the effect/point items, letters and the cristals you get from killing the enemies.