These characters have not shown up yet and probably won't, but a new Terrific Trio is referenced in Justice League Beyond #23 (May 2013), this time consisting of Earthmover, Zeta, and an elderly Plastic Man. According to the author, the idea - which was never fully established - was that the Terrific Trio became a Royal Flush Gang-type group that regularly changed members but always had the same basic make up.
These aren't new characters, however. Plastic Man has been appearing in the comics since the 40s, and Zeta and Earthmover first appeared in Batman Beyond. Earthmover first appeared in the episode of the same name, and his deal is that he was caught in a cave-in of an abandoned mine/toxic dump and died, but the toxic chemicals allowed him to create and control living dirtmen.
Zeta is more interesting to me, though. He is an infiltration/assassin robot created by the NSA that can mimic the look of anyone. He becomes self-aware, though, and decides he doesn't want to do that kind of work anymore. After his first appearance, "Zeta" (2000), which was great, he was given his own spinoff, The Zeta Project, and given a new, more humanoid look. He is therefore the protagonist of the only show in the DC Animated Universe not based on a pre-existing comic book.