Just a few things....totally agree with Carina here. While amoxycillin is fine for dogs you should only you use it after advice from your vet, don't be giving your dog stuff prescribed for humans, you could overdose them. Dosage of antibiotics is worked out relative to the weight of the animal. So giving a small dog (like a terrier) a human dose wouldn't be good.
Like Carina said, unless your dogs have got a secondary bacterial infection, there's not much point giving them antibiotics as the antibiotics don't work for viral infections, only the dogs immune system can fight a viral infection.
Also, $300???? This sounds very steep for a course of antibiotics for three dogs. If it's only antibiotics you're getting I think this is way too expensive, you should query why it's so dear. A course of antibiotics would be around £10 (at the most) times 3 then that's about £30 which is about $52, definately not $300