It never happened, but it should have.
While Robotech was first being broadcast, its creator, Carl Macek, was making plans for an all-new animation sequel, to be known as Robotech II: the Sentinels. The existence of this project, and its ultimate fate, is well-documented in general, and well-known to the Robotech RPG community in particular. Less well known is that Mr. Macek had plans for yet another sequel, set to take place after the events of the New Generation and Robotech II: the Sentinels.
Robotech III: the Odyssey would have taken place on board the SDF-3 after it misjumped on its return trip. In truth, the SDF-3 did make it back to Earth; but it arrived thousands of years too soon. Most of the details are unknown, beyond the facts that Mr. Macek intended for Lynn Minmei to ultimately end up as the mother of the original Zor and that the series would have ended with the arrival of the Visitor on Earth in the early 2000's. Beyond that, all that I have is speculation.