After the 3rd impact, the world was never the same. For the Tendo and Saotome move to Tokyo-3 to get away from the rising sea.
The Second Impact changed the world, but Nodoka and Genma survived it, marrying the following May. Ranma was born soon after, and when they were 5, they moved to Tokyo 3. Ranma went on his training trip with Genma and when he returned he was engaged to Akane, and went to school. Sounds like a normal life, but soon the Angels will change everything.