Yes you can watch Aku no Hado without watching the others. It is a side story that can be seenas a standalone too.
With the main series, the order is; 1. Ishi no Mayu, 2. Suishou no Kodou, and 3. Chou no Rikigaku. However the 3 dramas are concepted in a ways that you do not need to see the others to understand the story. So you can decide with which one you want to begin, if you do not want to follow the order.