The men broke into the house in Rouken Glen Road in Giffnock, near Glasgow, at about 17:30 on Monday.
They made off with cash and jewellery when members of the woman's family arrived at the house.
Two of the men were driven away in a dark grey Volvo driven by a fourth man. The third member of the gang was seen heading towards Eastwood Toll on foot.
The woman they threatened was unhurt but said by police to be "extremely shaken".
Officers are attempting to trace the men, who were all wearing dark clothing.
Det Con Craig Faskin said: "This was an extremely frightening ordeal for this woman who was within her own home and those responsible must be caught.
"Officers are still gathering information on this crime and establishing exactly what was stolen from the house and the exact circumstances surrounding the crime.
"Rouken Glen Road would have been very busy around the time of this robbery and I would urge any witnesses who have not yet spoken to police, or anyone with information that may assist our ongoing investigation to contact Giffnock Police Office."
A 69-year-old woman has been threatened by three masked men during a raid on her home.