Fish oils are terrific! But, that being said, there are differences in formulations. You can certainly give your dog fish oil for humans. You need to be careful about the mercury content in the fish oils you choose. Generally speaking, Salmon or Cod Liver oils have the same content as the 3V caps sold by your veterinarian. Some fish oil supplements have other things in them like EPA and DHA. If the formulation you have has those substances, please limit the amount to 180mg EPA and 120 mg DHA/10 lbs body weight daily. If the formulation you find has Vitamin A in it, you have to be careful not to overdo it as you can get Vitamin A toxicity.
The dose to use would depend upon the formulation you choose and if your dog is itchy or just has flaky skin.
You can purchase the fish oil caps at your health food store and if it is pure fish oil with nothing else added, use about 25% of the human dose for a 20 pound dog. You would need to break open the capsules and put it on the food in order to dose it this way.
Please let me know if you have further questions or if you need more information.