These Pokemon are exclusive to the Gold, Silver, and Crystal games and their remakes HeartGold and SoulSilver.
How to find them in Gold/Silver/Crystal/HeartGold/SoulSilver:
1. Beat the Elite Four: This unlocks the ability to roam the world freely.
2. Talk to the old man in the National Park: He will tell you that the legendary Pokemon are roaming the region.
3. Use the "Radio Tower" trick: This involves using the Radio Tower in Goldenrod City to track their locations. The specific frequencies each Pokemon uses are:
* Entei: 89.9 MHz
* Raikou: 93.7 MHz
* Suicune: 98.1 MHz
4. Find them using encounter chains: These are chains of Pokemon encounters that eventually lead to a legendary Pokemon.
If you want to catch these Pokemon, you'll need to transfer them from a compatible game like Gold, Silver, Crystal, HeartGold, or SoulSilver using the Game Boy Link Cable.