Gold/Silver/Crystal- Entei, Raikou, Suicune. Any route with grass. They cannot be found in water or caves. Suicune does not roam in Crystal. The Legends only appear in Johto.
Fire Red/Leaf Green-Entei, Raikou, Suicune. (Entei if your starter is Bulbasaur, Raikou if your starter is Squirtle , and Suicune if your starter is Charmander.] Any route with grass. Cannot be found in Water or Caves.
Ruby/Sapphire/Emerald- Latias, Latios. One or the other depending on what color you picked the Pokemon to have been on the TV when talking to the mom. Can be found anywhere in grass or water. No caves.
Diamond/Pearl- Mesprite and Cresselia. Routes with grass excluding water and caves.
Platinum-Mesprite, Cresselia, Zapdos, Moltres, and Articuno. All roam in grassy routes as well, not in water or caves.
Heart Gold- Entei, Raikou, Latias. All in grassy routes, no water or caves. Entei and Raikou are only in Johto. Latias is only in Kanto.
Soul Silver- Entei, Raikou, Latios. Same as Heart Gold. Entei and Raikou are only in Johto. Latios is only in Kanto.
Black/White 1- Tornadus (Black) Thundurus (White). At night before midnight on routes 9, 10, 11. Between midnight and morning, Routes 12, 13, 24, 15. Morning are routes 2, 3, 18. Lastly daytime are routes 4, 5, 16.
X and Y- Articuno, Zapdos, and Moltres all appear in grass.
Source- Experience... And https://bulbapedia.bulbagarden.net/wiki/Roaming_Pokémon#Generation_V_2
