better to take your dog to the vet and have the legs x-rayed. Your dog might have broken or dislocated a bone. Normally a dog will learn how to walk with 3 legs and will cope just fine.
Since your dog is still a puppy there might be a chance for the bones to heal as it is still growing.
为了更好地把你的狗到兽医及腿部X光检查. 你的狗有可能破裂或脱臼骨折. 通常一只狗,将学习如何走路3条腿和应付好了. 因为你的狗还是小狗可能会有一个机会,让骨头愈合as it is 仍在扩大.