A 3-year-old Long Island boy yesterday wandered out of his grandparents’ house unnoticed, got past a gate whose lock was broken and drowned in the family’s backyard pool, police said.

The tragic tot and his two siblings had just been dropped off at the Laurel Hollow home when his grandma realized he was missing, police said.

She found him in the pool, frantically pulled him out and called 911, but he died at Syosset Hospital.

Officers said the boy slipped into the garage through an open door and made his way to the yard, where a 4-foot fence surrounds the in-ground pool.

No charges will be filed, Nassau County cops said.

“It’s tragedy enough,” said Detective Lt. Kevin Smith.