I am really really sorry, but nobody can tell you how long you have to live.
3rd stage is pretty bad, as you probably know, and sometimes melanoma moves very fast.
The best thing you can do is to get your affairs in order, make a will and also a living will, to say whether you want to be kept alive at the end of your life at all costs, or whether you want to be allowed to drift away under large doses of pain relief. The do as the best consultant advises you.
If you get cancer in your liver, and the surgeons offer to operate ( the liver can regenerate itself, actually) you may have quite a few months left. If you get liver secondaries,and they are not operable, you haven't got more than a very few months, unless you are extremely lucky.