The Syrian Observatory for Human Rights said Al-Nusra had taken control of Sufuhun, Al-Fateera and Hizareen in Idlib, where the jihadist group has been battling other rebels.
The advances were the latest by Al-Nusra against moderate and Islamist rebels in the province after clashes that began a week ago.
The Observatory said the three villages had been under the control of various rebel groups including the Hazm movement, which has received US-made weapons.
Last week, Al-Nusra seized the town of Khan al-Subul from Hazm and the Idlib bastion of another Western-backed rebel grouping, the Syria Revolutionaries Front.
It remains unclear whether Al-Nusra has seized the US-made TOW anti-tank missiles received by Hazm.
Syrian moderate and Islamist rebels have in the past allied with Al-Nusra in battles against both the Syrian regime and the jihadist Islamic State group.
But since July, sporadic clashes have erupted between the Al-Qaeda affiliate and other rebel groups, particularly in Idlib.
The fighting began shortly after Al-Nusra's chief announced the group's intention to establish an Islamic "emirate," intended to rival the "caliphate" established by the Islamic State group.
Elsewhere, the Observatory, which relies on a network of sources on the ground, said clashes Thursday between regime forces and Islamist fighters in southern Syria killed at least 40 people on Thursday.
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The monitor said at least 26 members of the pro-regime National Defence Forces and 14 Islamist fighters, including from Al-Nusra, were killed in Beit Tima, a majority-Druze region.
There has been fighting between regime and rebel forces in the region, in southeastern Damascus province, for more than a year, but Thursday's toll was the highest in a single day since violence began there.
Syria's Druze, a heterodox Muslim community, have largely stayed on the sidelines of the country's bloody conflict and Beit Tima remains under regime control.
A Lebanese security source told AFP that 11 rebel fighters injured in the clashes had been prevented by Lebanese soldiers from crossing the mountainous border area to seek medical treatment.
The source said it was the first time that Lebanese troops had prevented injured Syrians from entering the country in that part of the border area.
Syrian refugees and both civilians and rebels wounded in fighting have regularly slipped across the porous border between the two countries.
In recent months, Lebanon has all but closed the border officially to incoming Syrian refugees however, with exceptions only for humanitarian reasons.
More than 1.1 million Syrian refugees are living in Lebanon, straining the country's limited resources and infrastructure.
Lebanon's army has also bolstered border security after bloody clashes between troops and jihadists coming from Syria in the border region earlier this year.