The Texas Department of Public Safety says the crash happened around 5 p.m. Sunday near Wallis, about 40 miles west of Houston.
The department says 68-year-old Fred Blair of Wallis, 32-year-old Matilda Blair and 3-year-old Andrew Blair all died in the crash. Trooper Erik Burse said Monday that the Blairs are related and live in Houston.
The glider was registered to the Greater Houston Soaring Association.
The Federal Aviation Administration says the glider went into a nose dive and didn't recover almost immediately after it was released from the tow aircraft.
Investigators are trying to determine what caused the accident.