S.T.
Any kind of cancer at stage 3 is tricky. Some melanomas are very agressive others are not. You really have to take notes at the physician's office to find out the specific type of melanoma then check reliable websites such as the Mayo Clinic, Cleveland Clinic, Sloan Kettering, American Cancer Society, etc
But even that will only give you information on average prognosis. My mom was diagnosed with ovarian cancer 6 years ago - and it went in to remission for 5 years. It recurred last year at this time and my mom is still here fighting it with chemo. Most people do not survive 6 years after ovarian cancer - but some people do. All of the statistics I read on reliable websites showed that my mom would have been long dead by now. She is still preparing her own meals, making her own bed, etc. the chemo is tough but so is she. ;o) She is clearly more weak and tired - but she gets to experience yet another Christmas and it seems, the birth of a couple more great grandchildren this coming Spring we hope.
Some of your loved one's prognosis will have to do with his/her outlook on life and desire to fight - but not entirely.
I know 2 close people who had melanoma - one person was diagnosed at stage 4 and was gone very quickly. Another was diagnosed at stage 2 with a few lesions about 8-9 years ago and hasn't had additional lesions since and she's super healthy.
The best thing to do as a loved one is to be supportive of whatever they decide to do. Standard is surgical removal, biospy & chemo & or radiation. Side effects of both are tough - but they have great new drugs that help manage them. Be available to do laundry, make meals, drive to the doctors' office, go grocery shopping, etc. Some people will ask for your help others won't. YOu may have to offer soemthing specific "Can I help with household stuff like laundry or cheet changing?" "I'm going grocery shopping - do you have a list?". you get the idea.
And pray. God does care about the details of your life and the details of your loved one. If you make yourself available and ask He will hear your prayers and answer them. Don't be afraid to say I'm not sure I believe - show me yourself, I don't think this is fair, tell me what I need to hear, etc. You will be srupirsed.
All the best mama - I just prayed for your loved one.