The first sentence is the worst, the second one is nearly perfect, and the third one needs some minor corrections.
1) The first problem is that the speaker is not addressing anyone. Who did the speaker tell? Me? You? Us? Be sure to clarify. Also, "before which all world faces" does not make much sense. I'm not exactly sure what you are trying to say, but I think what you should have is "The speaker told [us] about an ecological disaster that faces all the world."
2) The only mistakes here are small grammatical errors. You need to add a comma after the word "sudden", as well as capitalize the pronoun "I".
3) To sound more natural and avoid the repeated use of "in the", I would change it to the following: "There was a gothic-style cathedral in the center of the city that had been built in the 16th century."
Hope that helped!