if this is all you need then you can probably do it without my mod with some cheats (here 3 islands should be created, 0.33 is the size):
game.create_surface("custom_map", { terrain_segmentation="0.33", property_expression_names = { elevation = "0_17-island" }, starting_points={{0,0},{1000,0},{0,1000}}})
game.players[1].teleport({0,0}, "custom_map")
I have not tried it however. you can find the different options for the new surface at https://lua-api.factorio.com/latest/Concepts.html#MapGenSettings
