I think using a suspension fork won't work - 3 kg is way lower than the usual loads a suspension fork is designed to isolate shocks at any meaningful level (the mass on the fork would be at least an order of magnitude below the usual on a bike, so it would be hard to have enough deflection to do something useful at 3 kg without bottoming out all the time on a regular load - its just too far from the operating regime of the fork normally). In any case, if it were possible, it would likely be a very sensitive adjustment that would need to be made and maintained.
This problem has been studied to some extent in cameras for image stabilization on bicycles, where it is suggested to directly mount a camera on a bicycle and having soft tires with some suspension (on the bicycle).
The movie industry way of stabilizing a camera while it is moving is with a steadicam. Some photography enthusiasts have made home made steadicam type things which you may want to start with and modify to mount on a bicycle. Unlike a real steadicam unit, these probably need to be toughened up a bit for 3 kg since my guess for the weight of a DSLR with a small lens is closer to 1 kg - 1.5 kg. This thread also suggests a vibration plate.