Three cups of black coffee before my fasting blood test. potassium came in at 5.7. could this be the cause. i'm healthy otherwise.

Unlikely...: The opposite correlation is actually more likely. Intaking caffeine can actually lower your potassium level because of the diuretic effect of caffeine.

No. : Have the test repeated. Blood cells can break down and release potassium. If the specimen was drawn in a way that traumatized the red blood cells, or if it sat around a long time before being tested, it will have an artificially high potassium.