Alma 19:14

Now Ammon seeing the spirit of the Lord poured out according to his prayers upon the Lamanites, his brethren, which had been the cause of so much mourning among the Nephites, or among all the people of God, because of their iniquities and their traditions; and Ammon fell upon his knees, and began to pour out his soul in prayer and thanksgiving to God, for what he had done for his brethren; and he was also overpowered with joy; and thus they all three had sunk to the earth.

Ammon saw the Spirit of the Lord pour out on the Lamanites. He had prayed for this to happen. These were his brothers. They had caused so much sadness among the Nephites and all the people of God. This was because of their sins and their traditions. Ammon fell on his knees. He began to pray and thank God for what he had done for his brothers. He was also filled with too much joy. So all three of them had fallen to the ground.