No one wants to be part of this group. But unfortunately for Hippolyta, she became an official member when Theseus ditched her to marry Phaedra. Theseus actually forced Hippolyta to marry him against her will in the first place, but then he ditched her to marry Phaedra. When Hippolyta tried to get revenge by rounding up a posse of Amazons and attacking the wedding party, Theseus killed the Amazon queen. So yeah, that's about as dumped as a person can get.
Yet another victim of Theseus's jerkiness, Ariadne got dumped by the Athenian hero early on in his career. After Ariadne helped Theseus win his fame by conquering the Labyrinth and killing the Minotaur, he had the nerve to ditch her on an island and sail away. (Why is this guy a hero again?)
Medea was the sorceress wife of the hero Jason, who got famous by sailing off with the Argonauts and bringing home the Golden Fleece. Jason never would've made off with the Fleece without Medea's help, but just like Theseus, he decided to diss the woman who made him a household name. When Jason dumped her for a pretty young princess, though, Medea went haywire and killed her and Jason's sons. For a ton more on this nasty breakup, check out the super-old play Medea by Euripides.
Deinara was the final wife of Heracles, who tried to dump her for a pretty young thing named Iole. This definitely didn't work out too well for the big guy. To try to get her husband to love her again, Deinara gave him a robe that was covered with what she thought was a love potion. Unfortunately, instead of rekindling the flames of love, the potion started burning off Heracles's skin—gross. You can find all the details of Deinara's accidental revenge under Heracles: Later Adventures and Death.