HI,
Off Hand I don't think there is a commercially available clamp on type adapter for that hub but a machine shop could probably make one (a custom one off would probably be pretty expensive tho).
If you must use a Nexus 3 Sp hub and want to precisely mount the sprocket, there is a version of the Nexus Hub that is compatible w/ a Disc Brake (maybe you could trade yours in for one? or maybe just the case is different and you could buy the "shell" and put your internals in the disc brake version of the hub to save cost?).
Converting to a hub w/ a disc brake mount would allow you to bolt a sprocket onto the disc brake mount area..(as a side note: I did read somewhere, that the Shimano Nexus hub w/ the 3 speed disc brake may use the "new" style center lock method for securing the rotor as opposed to the "regular" 6 hole mounts....I would have to do some research on that but it is kinda late so I'll have to research that later BUT probably in either case a sprocket is easier to make than a clamp on type adapter).
Anyways, That may be an easier alternative than trying to make a clamp on type sprocket adapter or waiting for somebody to make one (I wonder how many 3 sp hubs are being run out there in the motorized bike world?).
Hope this helps you.
Andrew
PS - FWIW - Sturmey Archer makes a 5 speed hub w/ regular 6 hole disc brake mounts....Model is S-RK5(N)..