Fish Oil does work as an anti-inflammatory...However, in a 3 month old puppy, you need to rule out Mange &/or Staph infection....Both are common in young dogs......A simple skin scrape (From your Vet) will rule out Parasites (Mange) & he/she will be able to tell by looking if it's a Staph Infection.....
Both conditions must be treated early ( With proper medications) or the battle is awful!!! Please, see your Vet ASAP.......Karla
I don't know what the dose would be. I think you need to find out what is causing the rash. Food? Something he is coming into contact with, etc.