Place the chilled coconut milk (the entire can), frozen pineapple and frozen mango in the blender. Turn the blender on high (or smoothie mode) and blend the ingredients together until they're whipped and smooth. If you need to, slowly add the pineapple juice to the blender, just until you get the consistency you want. You may need a bit more or less pineapple juice, depending on the cream to water ratio in the coconut milk you used.
Serve your smoothie with straws on a hot day and enjoy!