Wide open, as high an ISO as you think you can get away with as far as noise goes at final use and that'll give you the highest shutter speed available. However, you will probably want a bit more DoF than wide open will allow, so you may have to drop the shutter speed a bit to gain a stop or 2 of aperture. IIRC, the ball's trajectory is fairly constant so you should be able to pre focus on it's plane of flight, then it's a matter of timing the release to catch the dogs in the right attitude or stage of catch. As Alastair said, knowing your kit could help us advise - little point saying f/2.8, ISO 3200, 1/750 if you're using a kit lens on n older body!