The three bodies were found Wednesday soon after an emergency call reported the shootings in Hacienda Heights, Calif., the San Gabriel Valley Tribune reported.
"The male shot the two women, before shooting himself, in an apparent murder-suicide," Los Angeles County Sheriff's Deputy Kim Manatt said.
Sheriff's Capt. Tim Murakami said officials found no signs of an intruder and no suspects were being sought.
Neighbors told the Tribune the couple cared for their daughter-in-law, 62, who was diagnosed with Alzheimer's about six years ago.
Authorities didn't release the victims' names but the couple's son, Jim Crabtree, identified the couple as Dan and Carol Crabtree and the woman as his wife, Rita Delehanty.
"It was a mercy killing," Jim Crabtree told the newspaper. "Dad killed his wife, my wife and then he killed himself. He saw the problems we were having."
He said his mother had severe arthritis and was in constant pain. He said his wife's condition was deteriorating rapidly and she required 24-hour care. His parents would care for her at their home while he worked.