I guess the crude answer would be "They die." In fact, they all suffer some kind fo violent death. Lupito is gunned down by a mob, Narciso is killed by Tenorio, & Florence drowns after hitting his head when diving into the lake. So that's "what happens" to all of them.
A better answer though would be that each teaches Tony something about life, the human condition, and his place in the world. Throughout the novel, Tony struggles with his destiny: What is he meant to become? Is he more Luna or Marez? Does God exist? Each of these characters helps Tony uncover the truths behind these questions.
Lupito is Tony's first experience with death. Watching the men gun him down changes his perspective on his father and the men of the town. It also leads him to question God and his plan: How could he leave a man clearly sick and hurt like that? It is this incident that sparks Ultima's return to his life, and it is she in whom he confides.
Narciso becomes Tony's confidant in a way as well. He shows Tony his garden, and explains a little more fo Lupito's pain. He also tries to fight Tenorio, & holds Ultima in the highest regard. Tony suspects that he knows of the golden carp as well, & he turns to Narciso for advice several times.
Finally, Florence leads Tony to challenge his ideas about god and religion. Tony admires Florence's resilience and strength, but is frightened by his defiance and open questioning. He wants to show Florence the golden carp, & is in fact on his way to do that when news of Florence's death reaches him. Tony is worried for Florence's eternal soul, & again wonders how God can be kind and vengeful at the same time.