Subungual melanoma is quite rare, it accounts for only 5% of melanoma diagnoses. It usually presents in only one nail so it would be very rare for it to occur in 3 nails at the same time. If you think of it as a mole under the nail - it's very rare for someone to have 2 melanomas on different parts of the skin so the same applies to melanoma under the nail.
All that being said, you should get it checked out by your GP to be on the safe side. They will probably do a phone consultation & ask you to email photos - they will then either be able to say it looks like a fungal infection or they will send the photos through to dermatology & it will be decided if you need a face to face appointment with them.
There are lots of posts on the forum from others who have had or are having the same problem - to read them just search Subungual melanoma in the 'Search Forum' bar at the top of the page.
Thank you so much for your detailed reply. My GP seems to have put my mind in a slightly easier position. He informed me that being 19 and having 3 nails affected the chances of subungual melanoma are infantesimally small. He has given me the community dermatologists number to book an appointment to figure out what the cause is, but it seems bening.
