It happened around 4 p.m. on the 2300 block of 77th Avenue in the city's Cedarbrook section.
According to police, the victim's grandson and his two friends were playing with a gun in a second-floor bedroom when the weapon discharged and hit the woman who was in a wheelchair on the first floor.
The woman was rushed to an area hospital after being shot in the head. She is currently listed as stable.
"This could have been very tragic. You got a grandmother sitting in her living room just enjoying the day and takes a bullet through the floorboards," said Captain John Walker with the Philadelphia Police Department.
It's still unclear who fired the weapon.
Police say all three teenagers are over the age of 18. One of the teens fled the scene before officers arrived. Two others are being questioned by police.
A handgun believed to be used in the shooting was recovered by officers.