Raikou, Entei, and Suicune are the three legandary pokemon that are missing in your pokedex. They are going to be running around Johto region. To have one, you need to have the following.
1. Finish your whole journey!
The three pokemon come out depending on the starter you chose. Here are the following. You'll realise that the starter are weaknesses to the three legandary pokemon. I you want to remember, here is the little chart.
Electic = Water
Fire = Grass
Water = Fire