Afflicted Graverobber, Ancient Soldier Varg, and Cerah the Old Explorer.
There are no item descriptions or dialogue regarding these three warriors. From their worn items and armor we can guess that they were all grave robbers come to the Sunken Kingdom in search of treasure. We know that the Sunken Kingdom was beset by grave robbers after the fall of the Sunken King, because of numerous clues like the rooms with empty chests and the presence of Flynn's Ring.
The Afflicted Graverobber wears the Alva set and wields Beserker Blades. The Afflicted Graverobber groans with a female voice when hit, so it cannot be Alva. We also know that Alva found enlightenment at Zullie's side, so if we were to encounter Alva, even a hollowed Alva, it would make sense that Zullie would be there too.
Ancient Soldier Varg wears Havel's Set and wields Havel's Dragontooth and Greatshield, items specific to Havel. How he came to be in possession of these items is unclear, but the name we are given (Varg) indicates that this is not Havel, just someone wearing his set and using his items.
Cerah the Old Explorer wears black leather armor, Lucatiel's Mask and the wields Dragonslayer Greatbow with an Estoc. The black leather armor is worn by those with ill intent. How Cerah came to possess a mask just like Lucatiel's is unclear, though it may indicate that the mask was not unique to Lucatiel and may have been a more common issue. The Dragonslayer Greatbow is a weapon wielded by the knights of Gwyn in the Age of Fire. How it came into Cerah's possession is unclear, but it may have been loot, picked up on Cerah the Old Explorer's travels.
Whoever these three are, it is evident that their quest ended in the Sunken Kingdom where they hollowed.