Never seen this myself, but the pup is only 3 weeks, and just starting to walk and find his feet, I would wait and see for another week, if the pup has a problem it will still be there in another week or two, if he were dragging his legs that is different, but if he is playing on his feet fine and just a little shaky when walking at present I would put that down to him getting used to walking this pup may just be taking longer to adjust, if the problem is still there at 4+ weeks call your vet for advice and the relavant tests can then be done. Fingers crossed it is not an on going problem.