The focal length is a property of the lens and not the camera. Thus, a 50mm lens is a 50mm lens, no matter what body it is attached to. What does change is the Field of View (FoV) of the lens-body combination. The smaller the sensor, the smaller the FoV for a given focal length.
Attaching a lens with a certain FL on a full-frame (35mm) camera gives a certain FoV. Attaching the same lens on a body with smaller sensor gives a FoV which is smaller by the same factor. This is called the crop-factor, or multiplier. The net effect is the same as of attaching a a lens of longer FL (by the same factor) on a full frame camera.
So, for Nikon APS-C cameras, the crop factor is 1.5. It means that a 100mm lens on an APS-C body produces the same FoV as a 150mm lens on a full frame body.
When you buy a lens of 70-300mm FL, it is 70-300 on every body you attach it. But the effect on D3000 would be as if it was 1.5 longer on a D3.