That would be Sagittarius A*, the supermassive black hole at the center of the Milky Way whose mass is about 4 million times that of the sun. BH3, on the other hand, is the most massive in its category of stellar black holes,
Astronomers have identified the most massive stellar black hole yet discovered in the Milky Way galaxy. This black hole was spotted in data from the European Space Agency's Gaia mission because it imposes an odd 'wobbling' motion on the companion star orbiting it.