The families were shot dead in their homes by members of a pro-regime militia, known as the Shabiha, which has been blamed for numerous atrocities since the conflict began 11 months ago.
The Ghantawi family, including a girl of 15 and two boys aged seven and five, lived near al-Firdaos square in the city. The al-Tirkawi family, with seven members, and the al-Zamal family, with eight members, both lived in the al-Nazirheen area of Homs. In total, the three incidents claimed 20 lives.
Rami Abdel Rahman, head of the Observatory, said the accounts of the killing had come from trusted primary sources in Homs.
"This is not the first time that the Shabiha have carried out killings like this," he said.
"When they come into an area, they kill people to cause fear and to say 'we can do more'."