It happened on Ackerman Avenue in Central Islip around 1:30 a.m. Sunday.
Police say 17-year-old Eugene Smith of Brooklyn, 19-year-old Luigi Casimir of Brentwood and 24-year-old Drew Watson of Central Islip were all standing outside the home when they were shot.
Smith was shot in the back and was taken to Southside Hospital in Bay Shore, where he was pronounced dead.
Casimir and Watson were also taken to Southside Hospital with non-life threatening injuries.
So far, police have not made any arrests.
If you have any information you are asked to call the Homicide Squad at (631) 852-6392, or anonymously to Crime Stoppers at 1-800-220-TIPS.