How to put a roof on to the 3rd floor and make the house look like 2 floors?

@tigerrun777  You can check this tutorial out for different ideas for roofing. https://www.carls-sims-4-guide.com/tutorials/building/roofs.php 

The image you shared is only a two story house with a gabled roof and you can add one tile rooms on the third floor or use the roof ornaments from the roof decoration section in the build buy catalogue.