My dog Daisy has IBS, liver disease, and allergies and she uses omega 3 with excellent results. We do not use omega 6 or omega 9 in our dogs.
We use 1 capsule of Nordic naturals. She is 8 lbs - we use a higher, therapeutic dose for her (and we do not follow the dosing on the bottle). This brand is purity tested to be free of things like lead and mercury and was recommended to us by our veterinary nutritionist.
Here is the one we use. We use the capsules.
Nordic Naturals Pet Products