I have white under fingernails-on about 3 nails. This is new. What is this?

Since I cannot see your fingernails, I will have to ask you a few questions.
Are these white spots on the fingernails or is the entire area under the fingernails white?
White spots most often develop as a result of mild to moderate trauma to your nail. If you can't think of anything that would have injured your nail, consider the fact that nails grow very slowly, so the injury may have occurred weeks before the spots ever appeared.

One of the most common causes for these little white spots, which can show up on both fingernails and toenails, is a condition called leukonychia, which is white discoloration of the nail plate.

If all of your nails are showing white spots or streaks, the leukonychia could be related to another more serious condition such as anemia, cardiac disease, cirrhosis, diabetes or kidney disease.

Hope this information answers your question.
Maryann