Prepare cake according to package directions using three 9" can pans. Bake and cool completely.
To prepare filling: mix pineapple, sugar, butter and flour in medium saucepan. Cook and stir over medium heat until mixture thickens. Remove from heat and add coconut. Place mixture in fridge and cool. Spread mixture on top of each layer, including top layer. Frost sides of cake with whipped frosting.