A class 3 nebula was a type of nebula.
In 2375, Derran Tal detected a class-3 nebula from the Varro generational ship. Harry Kim secretly took her there in person, using one of USS Voyager's shuttlecraft. They found that there was a subspace vacuole near the center of the nebula, acting as a gravitational anchor for the surrounding gas. (VOY: "The Disease")