Well, basically what you're talking about is an extension tube, which is a hollow collar that mounts between the back of the lens and the camera body. However, by moving the lens away from the body (and 3" is about 76 mm), you're going to be changing the point of focus considerably - making your 'normal' lens into an extreme macro.
Without more information on the project, it's hard to be able to advise you on what you're trying to do. If you want to move the lens 3" in front of the camera body and have it focus and operate normally, the answer is no.
Mike.