Two men and a woman were shot dead on Monday in separate incidents in and around the flashpoint central city of Homs and in the north western city of Idlib, the London-based Syrian Observatory for Human Rights said.
They were killed by security forces manning checkpoints, the Observatory's chief Rami Abdel Rahman told AFP in Nicosia on Tuesday.
One man was killed as he was heading to work in Homs, a bastion of anti-regime dissent where the army has been conducting a bloody crackdown since last week, he said.
Another man was shot dead on Monday night when security forces manning a checkpoint in Talbissah outside Homs opened fire, Abdel Rahman added.
The woman was riding a motorcycle with her husband and child in Idlib, and they had just driven past a security checkpoint when she was killed, he said.
Abdel Rahman also reported "heavy gunfire" on Tuesday in Homs.
Meanwhile, the National Organisation for Human Rights accused the Syrian authorities of turning the country into a "huge prison" by arresting hundreds of people, despite lifting emergency rule in April.
"Syria has become a huge prison," NOHR chief Ammar Qorabi said in a statement on Tuesday which also gave the names of some 250 people who have been arrested nationwide in the past few weeks.
Several other people have also been detained but their names could not be confirmed, the statement said.
"The Syrian authorities are still pursuing arbitrary arrests of political activists, academics and civilians and storming homes as Syrian civilians continue to disappear by the hundreds," it said.
"This is a flagrant violation of the basic rights that are guaranteed by the Syrian constitution, despite the lifting of the state of emergency."
Eight protesters were among those arrested on July 20 in Damascus, while three lawyers were detained in Aleppo after taking part in a sit-in at the Bar Association and being "beaten with electric cables and chairs."
In April embattled President Bashar al-Assad signed decrees lifting five decades of draconian emergency rule and abolishing the feared state security courts, but activists say the wave of arrests has not abated.
The Observatory also reported on Tuesday that security forces had swooped on the town of Tal Rafaat in Aleppo province at dawn on Sunday, arresting around 25 people.
"Some of them were taken as hostages to press for the surrender of other family members wanted by the authorities," the statement said.