I'm 15 boy.I masturbate once a day.. some time thrice a day.is it harmfuk?

Not harmful as such, but too much can cause serious issues.

No masturbation isn't harmful, however it does carry a few responsibilities, such as always do this in Private, if in your bedroom close the door.

Don't leave your ejaculate fluids all over the place, clean up once you've done, put any clothing with sperm on it into the laundry basket.

Use toilet paper or tissues. Not your bed sheets.

Respect that others aren't going to want to see you doing this its personal and private, also once you're done make sure you wash your genitals and of course your hands.

Not harm ful but it might if your mom catches you.