3 Nephi 8:3

and the people began to look with great earnestness for the sign which had been given by the prophet Samuel, the Lamanite; yea, for the time that there should be darkness for the space of three days, over the face of the land.

The people started looking for the sign that Samuel the Lamanite had told them about. They watched for three days of darkness over the land.

People began watching anxiously for the sign Samuel the Lamanite had prophesied—three days of darkness covering the land.