Gallo, based in Modesto, CA, has about 50 labels with multiple brands being made at the same facility. Here are the U.S. brands. Even the casual wine shopper will recognize many of these. And you probably didn't even know you were buying Gallo. Besides the Gallo name they own Barefoot, Carlo Rossi, MacMurray Ranch, Turning Leaf, and a bunch more.
Constellation Brands, headquartered in New York State, is sort of a drinks holding company in that they've bought up wineries, but leave them alone, more or less, to make their products. That is, their wineries are more autonomous than Gallo's. Constellation owns about 50 wineries listed here including Clos du Bois, Ravenswood, Robert Mondavi.
The Wine Group, out of Livermore, CA in the Bay Area, has about 15, mostly less expensive labels, like Almaden, Cupcake, and Franzia. Their brands.
Gallo is privately owned by the family and still quite secretive. Constellation is a public company. The Wine Group is privately held.
Constellation plays in the premium wine brands arena more than Gallo or The Wine Group though Gallo is making a push in that direction with some recent acquisitions. Constellation owns several brands in Sonoma County. Gallo has a huge operation in the county, too. You'd probably never notice it driving by on Dry Creek Road, but there is a huge tank farm and underground storage sitting back off the road. Why do we call it a tank farm? Here it is from Google Earth, surrounded by vineyards.