Jardaan were approaching Type 3 by operating on the scale of an entire cluster with a black hole in the center of it - a dwarf galaxy.
Leviathans were operating on the scale of the entire Milky way galaxy. They created Reapers who basically turned Milky Way into Living Matter Factory, which they harvested to build more Reapers. Living Matter production requires solar and planetary energy and as I said they were operating on the scale of the entire galaxy, effectively harnessing its energy. Also their Mass Effect technology allows them to approach Black Holes and Collectors were using one for a base of operations. And that's just based off what we know about them.