How to substitute unsweetened coconut for sweetened coconut?
Assuming you want to keep the recipe as close as possible to the original, you could simply replace the 3 cups of sweetened shredded coconut by sweetening yourself 3 cups of unsweetened shredded coconut.
Here, as an example, this is a recipe to sweeten 1 cup of unsweetened shredded coconut. You can find the details and adjust the recipe to make 3 cups. Keep in mind that it is designed for organic unsweetened shredded coconut, so the quality of your coconut should play an important role in the result. As a quick summary of the recipe they provide this image

PS: If you plan on storing some, remember to allow to dry as the recipe says.