The Third Island is Connected to a Forest which contains Many Items and Wild Pokémon. However there is a girl here who is under attack by a Hypno and you must rescue her to get back to Cinnibar:

Other Than that Three Island is a quiet Resort town with Wild Pokémon
But After you beat the Elite 4, Right outside the Port is a cave which when you follow through leads you to a Patch of Grass filled with Johto Pokémon

Location Map of Three Island

Hover to get area

Three Isle Port Three Island Bond Bridge Berry Forest

Wild Pokémon on Three Island

Here are the Pokémon you will find in the wild on Three Island:

In The Grass
Oddish Common
Gloom Rare
Bellsprout
Common
Weepinbell
Rare
Meowth Common
Common
Persian Rare
Rare
Venonat Common
Common
Venomoth Rare
Rare
Psyduck Uncommon
Slowpoke
Uncommon
Pidgey Common
Common
Pidgeotto Rare
Rare
Dunsparce Common
Common
In The Water
Krabby Common
Common
Tentacool Common
Common
In The Forest
Pidgey Common
Common
Pidgeotto Rare
Rare
Drowzee Common
Common
Hypno Rare
Rare
Psyduck Uncommon
Slowpoke
Uncommon

Thats all on Three Island...Click Here for Four Island

 
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