You are mistaken, the maintenance per city on another landsmass is ~<.5 gpt when 2 cities are present, and ~1.25 gpt when 3 cities are present. That third city will cost 2.75 gpt, just counting overseas maintenance. Now add palace distance and amount of cities, etc. I believe the island is too bad for 3 cities, actually. Lets just settle Plains tile and then take barb city once it has grown pop/culture.