Realms of Arkania: Star Trail

To assign heroes to a night watch, you need to set up a camp. You can do this anywhere outside of a building, including at the start of the game. Head outside and press to bring up the action wheel. Towards the bottom left is a camp icon. Select this, and then press again to bring up the next wheel. Directly to the left is a shield icon. This is how you set up a hero for the night watch.

To use the same hero all three times, you simply need to hit the same character icon for each round of the watch. Once you do that, then choose the icon to sleep. Do this for a few hours.

Once that activity is done, you will automatically exit the camp and the achievement will unlock.