The west portion of Three Isle Port is just a pathway or connector to Three Island from the Seagallop Ferry dock. There is a cave entry that leads to Three Isle Path that ends on the eastern side of Three Isle Port. Most of the West segment is made up of sandy ground.
The east portion cannot be accessed until after the player has defeated the Elite Four and has received the National Pokédex, because the Three Isle Path initially incomplete. After it is finished, the player can go through Three Isle Path and reach the east segment. The east portion seems to be a secluded rock cliff with a small patch of tall grass. Here, the player can find Dunsparce.