The Devon club recorded a 3-1 Johnstone's Paint Trophy triumph in September and then followed that up with another 3-1 victory in the first round of the FA Cup last month.
Torquay started on fire with on-loan Wycombe forward Zebroski rattling the foot of a post in the opening minute. And he was not to be denied soon after, scoring with a brilliant 25-yard shot on the run that skipped up off the wet pitch and in off a post past Robins keeper Scott Brown.
Opposite number Michael Poke kept out Drissa Diallo's header but it was United who doubled their advantage with Thomson playing a key part. He embarked a meandering run and when he was finally tackled, the ball broke to Rendell on the left side of the penalty area who swept the ball home.
United started the second half as they began the first, on the front foot. And when Zebroski released Thomson, the winger set his sights on goal and gained his just reward for a man-of-the-match performance by firing home the third in the 48th minute.