The three heroes of the Prophecy aren't three individuals, but one individual on three stages of their journey. A prince (female) becomes a cage for a human soul, before finding love and having hope crossed on her heart, but it ends in tragedy as she ends up alone in deepest dark.
Toriel conspired with the creator of the vessels to create someone that could fulfill the conditions of the prophecy. This vessel was adopted by the Dreemurr family and named Kris, and was secretly guided by Toriel, who believed it was the best way to save everyone from the prophesied Roaring. She did not anticipate Kris actually being able to bond not just with her family, but with the Holidays as well, becoming a person in her own right instead of just an empty creation.
Dess is the only person who figured out Toriel's plans and was disgusted by Toriel casually sacrificing Kris. She led Kris into the shelter hoping to find a way to break it, but ended up being lost in the dark while Kris escaped.
The broken panel of the prophecy is phrased along the lines of "the girl destroys the cage." To Susie this makes her thing that she needs to kill Kris, but originally it referred to the hero fully unleashing the human soul to permanently banish the dark worlds. Unfortunately, this has to happen in the deepest dark, locking the hero out of the light world and leaving her alone in deepest dark. This is why Susie and the power of friendship will literally save the day, because simply by being with Kris in the dark will be enough to break the prophecy.