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

Hover to get area

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