It ends with a cliffhanger... Again!
Some ideas, puzzles and story elements are fun and interesting, but they are few and far between.
If you want to know what happened after the cliffhanger in the second game, don't bother. This one ends with an even bigger cliffhanger. If that wasn't bad enough, the story of Simon 3d ignores parts of Simon 2 ending.
While Simon 4 does exist and its story does acknowledge the existence of Simon 3D it does not continue the Simon 3D storyline. It makes fun of it and simply ignores the fact that Simon 3D has no ending.
Simon 1 and 2 are classics, even if part 2 doesn't end the storyline.
Simon 3D is an overambitious, half baked mess that has no reason to exist. It's not fun, it doesn't have a complete story and it shows bugs all the time.
The only reason to play it is either to see how much a franchise can fall or to see some of the craziest puzzles in existence.