3 days = 3 trips underground. Day 1 was the Catacombs at San Gennaro. They're all empty holes now and the bodies relocated. During WWII this was used as a safety bunker for air raids. The first level started as early as the 3rd century and additions kept happening. You can't explore it all for safety reasons but the guided tour is about 45 minutes. catacombs-of-san-gennaro-naples
Day 2 - Fontanelle Cemetery. Want to adopt a skull for luck? Do it here. Win at gambling? Make your skull happy. Bring it stuff. Preferrably brightly colored. I guess to offset how dark and grey it is in here. The caverns go back and light is limited. 1000's of bones are here, the last resting place of the poor. Kind of spooky, creepy, and fascinating. Click here for info
I wasn't kidding - adopt away!