I don't know about the D3100 but the D90 and D60 both have a decent action shot settings. I use it when I'm feeling lazy.
One thing I do, that I don't see mentioned is that I sit on the ground and usually hold the camera on the ground. It is MUCH harder to get decent action shots of dogs as small as ours are with nice backgrounds. If I can, I try to set it up so that the dogs are up a little higher than I am. If it's sunny, sun goes to my back.
I also use multiple toys if I'm going for ball shots. It is very easy to set up a shot by throwing one toy right then the other left. Mia will run for the first then sprint over to the second. although lately she's figured out my plan and is trying to pre-empt me to get to ball #2. So I may rethink that a bit. But generally ball #1 is to get her 'positioned' because she's usually right on top of me. Then I use the rest of the balls to get the shots I want.
Mia and Summer