Melanoma lesion on outside - only 3 years old

Firstly, take a deep breath. There is nothing that you can do today, or tomorrow, except act normal around your dog.

Not quite two years ago, Finn, my now going on 4 year old male Vizsla, had a biopsy done on a growth on his neck. Same diagnosis as you got, a cancerous melonoma growth. Scared the beejeezus out of me also.
He was scheduled for surgery, the melanoma removed, with a what is known as margin. ( They take extra skin around the growth to minimize leaving any cells behind.) I also elected to have him neutered at the same time only to have go through the anesthesia once. Complete xrays were done also at the time. They also showed "anomolies". They turned out to be "fatty cells".
Finn had follow up blood tests done, and once healed from the surgeries went to a Veterinary Oncologists in Enfield CT. They did more tests, took some more pictures, counseled us on potential outcomes and treatment protocols. In Finn's case, nothing extra was needed and he was given a clean bill of health, but he still gets a more intensive exam twice yearly.
All of this was done during the height of Covid Protocol's nationwide, which definitely added to my anxiety level.

As hard as it will be, just try and relax. Don't start reading internet horror stories. Most are outdated and the treatment protocols have been replaced. Veterinary Oncology has made exponential gains in the past decade on treatments. It's a field all it's own. As with humans.

Mike