I have a black vertical line on 3 nails?

Splinters: These could be what doctors call "splinters" , but not the kind you are familiar with. They could be small blood clots that got caught there after circulating around in the blood. Could be a sign of heart valve problem. You need to see a doctor about this. Emergency room not necessary if no other symptoms, i.e., chest pain, feverishness, shortness of breath, major fatigue, etc.

Melanonychia: Melanonychia is brown or black pigmentation of the nail unit. Melanonychia commonly presents as pigmented band arranged lengthwise along the nail unit, and this presentation is known as longitudinal (lm) melanonychia or melanonychia striata. The most important cause of melanonychia is subungual melanoma, although a variety of other causes includes systemic disorders, trauma, inflammatory disorders.